Steel sheet, coated steel sheet, and methods for manufacturing same

ABSTRACT

A steel sheet having TS of 980 MPa or more and YR of 68% or more is obtained by providing a predetermined chemical composition and a predetermined steel microstructure, where an average aspect ratio of crystal grains of each phase (polygonal ferrite, martensite, and retained austenite) is 2.0 or more and 20.0 or less, wherein the polygonal ferrite has an average grain size of 4 μm or less, the martensite has an average grain size of 2 μm or less, the retained austenite has an average grain size of 2 μm or less, and a value obtained by dividing a Mn content in the retained austenite in mass % by a Mn content in the polygonal ferrite in mass % equals 2.0 or more.

TECHNICAL FIELD

This disclosure relates to a steel sheet, a hot-dip galvanized steelsheet, a hot-dip aluminum-coated steel sheet, and an electrogalvanizedsteel sheet, and methods for manufacturing the same, and in particularto a steel sheet with excellent formability and hole expansionformability and high yield ratio that is preferably used in parts in theindustrial fields of automobiles, electronics, and the like.

BACKGROUND

In recent years, enhancement of fuel efficiency of automobiles hasbecome an important issue from the viewpoint of global environmentprotection. Consequently, there is an active movement to reduce thethickness of automotive body components through increases in strength ofsteel sheets as automotive body materials, and thereby reduce the weightof automotive body itself.

In general, however, strengthening of steel sheets leads todeterioration in formability, causing the problem of cracking duringforming. It is thus not simple to reduce the thickness of steel sheets.Therefore, it is desirable to develop materials with increased strengthand good formability. In addition to good formability, steel sheets witha tensile strength (TS) of 980 MPa or more are required to have, inparticular, enhanced impact energy absorption properties. To enhanceimpact energy absorption properties, it is effective to increase yieldratio (YR). The reason is that a higher yield ratio enables the steelsheet to absorb impact energy more effectively with less deformation.

Moreover, in the case of using a steel sheet in an automotive body,stretch flanging according to the shape of the automotive body isperformed, so that excellent hole expansion formability is required,too.

For example, JPS61157625A (PTL 1) proposes a high-strength steel sheetwith extremely high ductility having a tensile strength of 1000 MPa orhigher and a total elongation (EL) of 30% or more, utilizing deformationinduced transformation of retained austenite.

In addition, JPH1259120A (PTL 2) proposes a high-strength steel sheetwith well-balanced strength and ductility that is obtained from high-Mnsteel through heat treatment in a ferrite-austenite dual phase region.

Moreover, JP2003138345A (PTL 3) proposes a high-strength steel sheetwith improved local ductility that is obtained from high-Mn steelthrough hot rolling to have a microstructure containing bainite andmartensite after subjection to the hot rolling, followed by annealingand tempering to cause fine retained austenite, and subsequentlytempered bainite or tempered martensite in the microstructure.

CITATION LIST Patent Literature

PTL 1: JPS61157625A

PTL 2: JPH1259120A

PTL 3: JP2003138345A

SUMMARY Technical Problem

The steel sheet described in PTL 1 is manufactured by austenitizing asteel sheet containing C, Si, and Mn as basic components, and subjectingthe steel sheet to a so-called austempering process whereby the steelsheet is quenched to and held isothermally in a bainite transformationtemperature range. During the austempering process, C concentrates inaustenite to form retained austenite.

However, while a high concentration of C beyond 0.3% is required for theformation of a large amount of retained austenite, such a high Cconcentration above 0.3% leads to a significant decrease in spotweldability, which may not be suitable for practical use in steel sheetsfor automobiles. Additionally, the main objective of PTL 1 is improvingthe ductility of steel sheets, without any consideration for the holeexpansion formability, bendability, or yield ratio.

PTLs 2 and 3 describe techniques for improving the ductility of steelsheets from the perspective of formability, but do not consider thebendability, yield ratio, or hole expansion formability of the steelsheet.

To address these issues, it could thus be helpful to provide a steelsheet, a hot-dip galvanized steel sheet, a hot-dip aluminum-coated steelsheet, and an electrogalvanized steel sheet that are excellent informability and hole expansion formability with TS of 980 MPa or moreand YR of 68% or more, and methods for manufacturing the same.

Solution to Problem

To manufacture a high-strength steel sheet that can solve the aboveissues, with excellent formability and hole expansion formability aswell as high yield ratio and high tensile strength, we made intensivestudies from the perspectives of the chemical compositions andmanufacturing methods of steel sheets. As a result, we discovered that ahigh-strength steel sheet with high yield ratio that is excellent informability such as ductility and hole expansion formability can bemanufactured by appropriately controlling the chemical composition andmicrostructure of steel.

Specifically, a steel sheet that has a steel composition containing Mn:more than 4.20 mass % and 6.00 mass % or less, with the addition amountsof other alloying elements such as Ti being adjusted appropriately, ishot rolled to obtain a hot-rolled sheet. The hot-rolled sheet is thensubjected to pickling to remove scales, retained in a temperature rangeof [Ac₁ transformation temperature+20° C.] to [Ac₁ transformationtemperature+120° C.] for 600 s to 21,600 s, and optionally cold rolledat a rolling reduction of less than 30% to obtain a cold-rolled sheet.Further, the hot-rolled sheet as annealed after the hot rolling or thecold-rolled sheet is retained in a temperature range of [Ac₁transformation temperature+10° C.] to [Ac₁ transformationtemperature+100° C.] for 20 s to 900 s, and subsequently cooled.

Through this process, the hot-rolled sheet or the cold-rolled sheet hasa microstructure that contains, in area ratio, 5% or more and 50% orless of polygonal ferrite, 10% or more of non-recrystallized ferrite,and 15% or more and 30% or less of martensite, and, in volume fraction,12% or more of retained austenite, where the average aspect ratio ofcrystal grains of each phase (polygonal ferrite, martensite, andretained austenite) is 2.0 or more and 20.0 or less, the polygonalferrite has an average grain size of 4 μm or less, the martensite has anaverage grain size of 2 μm or less, and the retained austenite has anaverage grain size of 2 μm or less. Moreover, the microstructure of thehot-rolled sheet or the cold-rolled sheet can be controlled so that avalue obtained by dividing a Mn content in the retained austenite (inmass %) by a Mn content in the polygonal ferrite (in mass %) equals 2.0or more, making it possible to obtain a volume fraction of 12% or moreof retained austenite stabilized with Mn.

This disclosure has been made based on these discoveries.

Specifically, the primary features of this disclosure are as describedbelow.

1. A steel sheet comprising: a chemical composition containing(consisting of), in mass %, C: 0.030% or more and 0.250% or less, Si:0.01% or more and 3.00% or less, Mn: more than 4.20% and 6.00% or less,P: 0.001% or more and 0.100% or less, S: 0.0200% or less, N: 0.0100% orless, and Ti: 0.005% or more and 0.200% or less, and optionally furthercontaining, in mass %, at least one selected from the group consistingof Al: 0.01% or more and 2.00% or less, Nb: 0.005% or more and 0.200% orless, B: 0.0003% or more and 0.0050% or less, Ni: 0.005% or more and1.000% or less, Cr: 0.005% or more and 1.000% or less, V: 0.005% or moreand 0.500% or less, Mo: 0.005% or more and 1.000% or less, Cu: 0.005% ormore and 1.000% or less, Sn: 0.002% or more and 0.200% or less, Sb:0.002% or more and 0.200% or less, Ta: 0.001% or more and 0.010% orless, Ca: 0.0005% or more and 0.0050% or less, Mg: 0.0005% or more and0.0050% or less, and REM: 0.0005% or more and 0.0050% or less, with thebalance consisting of Fe and inevitable impurities; and a steelmicrostructure that contains, in area ratio, 5% or more and 50% or lessof polygonal ferrite, 10% or more of non-recrystallized ferrite, and 15%or more and 30% or less of martensite, and that contains, in volumefraction, 12% or more of retained austenite, where an average aspectratio of crystal grains of each of the polygonal ferrite, themartensite, and the retained austenite is 2.0 or more and 20.0 or less,wherein the polygonal ferrite has an average grain size of 4 μm or less,the martensite has an average grain size of 2 μm or less, the retainedaustenite has an average grain size of 2 μm or less, and a valueobtained by dividing a Mn content in the retained austenite in mass % bya Mn content in the polygonal ferrite in mass % equals 2.0 or more.

2. The steel sheet according to 1., wherein the steel microstructurefurther contains, in area ratio, 2% or more of ε phase with an hcpstructure.

3. The steel sheet according to 1. or 2., wherein the retained austenitehas a C content that satisfies the following formula in relation to theMn content in the retained austenite:

0.04*[Mn]+0.056−0.180≤[C]≤0.04*[Mn]+0.056+0.180

where

[C] is the C content in the retained austenite in mass %, and

[Mn] is the Mn content in the retained austenite in mass %.

4. A coated steel sheet comprising: the steel sheet according to any oneof 1. to 3.; and one selected from a hot-dip galvanized layer, agalvannealed layer, a hot-dip aluminum-coated layer, and anelectrogalvanized layer.

5. A method for manufacturing a steel sheet, the method comprising:heating a steel slab having the chemical composition according to 1.;hot rolling the steel slab with a finisher delivery temperature of 750°C. or higher and 1000° C. or lower to obtain a steel sheet; coiling thesteel sheet; then subjecting the steel sheet to pickling to removescales; retaining the steel sheet in a temperature range of [Ac₁transformation temperature+20° C.] to [Ac₁ transformationtemperature+120° C.] for 600 s to 21,600 s; optionally cold rolling thesteel sheet at a rolling reduction of less than 30%; and then retainingthe steel sheet in a temperature range of [Ac₁ transformationtemperature+10° C.] to [Ac₁ transformation temperature+100° C.] for 20 sto 900 s and subsequently cooling the steel sheet, to manufacture thesteel sheet according to any one of 1. to 3.

6. The method according to 5., wherein a value obtained by dividing avolume fraction of the retained austenite after performing tensileworking with an elongation value of 10% by a volume fraction of theretained austenite before the tensile working equals 0.3 or more.

7. The method according to 5., comprising after the cooling, eithersubjecting the steel sheet to one selected from hot-dip galvanizingtreatment, hot-dip aluminum coating treatment, and electrogalvanizingtreatment, or subjecting the steel sheet to hot-dip galvanizingtreatment and then to alloying treatment at 450° C. or higher and 600°C. or lower, to manufacture the coated steel sheet according to 4.

Advantageous Effect

According to the disclosure, it becomes possible to provide ahigh-strength steel sheet with excellent formability and hole expansionformability and high yield ratio that exhibits TS of 980 MPa or more andYR of 68% or more. High-strength steel sheets according to thedisclosure are highly beneficial in industrial terms, because they canimprove fuel efficiency when applied to, for example, automobilestructural parts, by a reduction in the weight of automotive bodies.

BRIEF DESCRIPTION OF THE DRAWING

In the accompanying drawings:

FIG. 1 illustrates the relationship between the working ratio of tensileworking and the volume fraction of retained austenite; and

FIG. 2 illustrates the relationship between the elongation of each steelsheet and the value obtained by dividing the volume fraction of retainedaustenite remaining in the steel sheet after subjection to tensileworking with an elongation value of 10% by the volume fraction ofretained austenite before the tensile working.

DETAILED DESCRIPTION

The following describes the present disclosure in detail.

First, the reasons for limiting the chemical composition of the steel tothe aforementioned ranges in the present disclosure are explained. The %representations below indicating the chemical composition of the steelor steel slab are in mass % unless stated otherwise. The balance of thechemical composition of the steel or steel slab is Fe and inevitableimpurities.

C: 0.030% or More and 0.250% or Less

C is an element necessary for causing a low-temperature transformationphase such as martensite to increase strength. C is also a usefulelement for increasing the stability of retained austenite and theductility of steel. If the C content is less than 0.030%, it isdifficult to ensure a desired area ratio of martensite, and desiredstrength is not obtained. It is also difficult to guarantee a sufficientvolume fraction of retained austenite, and good ductility is notobtained. On the other hand, if C is excessively added to the steelbeyond 0.250%, hard martensite excessively increases in area ratio,which causes more microvoids at grain boundaries of martensite andfacilitates propagation of cracks during bend test and hole expansiontest, leading to a reduction in bendability and stretch flangeability.If excessive C is added to steel, hardening of welds and theheat-affected zone (HAZ) becomes significant and the mechanicalproperties of the welds deteriorate, leading to a reduction in spotweldability, arc weldability, and the like. From these perspectives, theC content is 0.030% or more and 0.250% or less. The C content ispreferably 0.080% or more. The C content is preferably 0.200% or less.

Si: 0.01% or More and 3.00% or Less

Si is an element that improves the strain hardenability of ferrite, andis thus a useful element for ensuring good ductility. If the Si contentis below 0.01%, the addition effect is limited. Thus the lower limit is0.01%. On the other hand, excessively adding Si beyond 3.00% not onlyembrittles the steel, but also causes red scales or the like todeteriorate surface characteristics. Therefore, the Si content is 0.01%or more and 3.00% or less. The Si content is preferably 0.20% or more.The Si content is preferably 2.00% or less.

Mn: More than 4.20% and 6.00% or Less

Mn is one of the very important elements for the disclosure. Mn is anelement that stabilizes retained austenite, and is thus a useful elementfor ensuring good ductility. Mn can also increase the strength of thesteel through solid solution strengthening. In addition, concentrationof Mn in retained austenite can ensure obtaining 2% or more of ε phasewith an hcp structure, and furthermore, guarantee the volume fraction ofretained austenite being as high as 12% or more. These effects can beobtained only when the Mn content in steel is more than 4.20%. On theother hand, excessively adding Mn beyond 6.00% results in a rise incost. From these perspectives, the Mn content is more than 4.20% and6.00% or less. The Mn content is preferably 4.80% or more. The Mncontent is preferably 6.00% or less.

P: 0.001% or More and 0.100% or Less

P is an element that has a solid solution strengthening effect and canbe added depending on the desired strength. P also facilitates ferritetransformation, and thus is also a useful element for forming amulti-phase structure in the steel sheet. To obtain this effect, the Pcontent in the steel sheet needs to be 0.001% or more. However, if the Pcontent exceeds 0.100%, weldability degrades and, when a galvanizedlayer is subjected to alloying treatment, the alloying rate decreases,impairing galvanizing quality. Therefore, the P content is 0.001% ormore and 0.100% or less. The P content is preferably 0.005% or more. TheP content is preferably 0.050% or less.

S: 0.0200% or Less

S segregates to grain boundaries, embrittles the steel during hotworking, and forms sulfides to reduce the local deformability of thesteel sheet. Therefore, the S content is 0.0200% or less, preferably0.0100% or less, and more preferably 0.0050% or less. Under productionconstraints, however, the S content is preferably 0.0001% or more.Therefore, the S content is preferably 0.0001% or more and 0.0200% orless. The S content is more preferably 0.0001% or more. The S content ismore preferably 0.0100% or less. The S content is further preferably0.0001% or more. The S content is further preferably 0.0050% or less.

N: 0.0100% or Less

N is an element that deteriorates the anti-aging property of the steel.The deterioration in anti-aging property becomes more pronounced,particularly when the N content exceeds 0.0100%. Accordingly, smaller Ncontents are more preferable. However, under production constraints, theN content is preferably 0.0005% or more. Therefore, the N content ispreferably 0.0005% or more and 0.0100% or less. The N content is morepreferably 0.0010% or more. The N content is more preferably 0.0070% orless.

Ti: 0.005% or more and 0.200% or less Ti is one of the very importantelements for the disclosure. Ti is useful for achieving strengthening byprecipitation of the steel. Ti can also ensure a desired area ratio ofnon-recrystallized ferrite, and contributes to increasing the yieldratio of the steel sheet. Additionally, making use of relatively hardnon-recrystallized ferrite, Ti can reduce the difference in hardnessfrom a hard secondary phase (martensite or retained austenite), and alsocontributes to improving stretch flangeability. These effects can beobtained when the Ti content is 0.005% or more. On the other hand, ifthe Ti content in the steel exceeds 0.200%, hard martensite excessivelyincreases in area ratio, which causes more microvoids at grainboundaries of martensite and facilitates propagation of cracks duringbend test and hole expansion test, leading to a reduction in thebendability and stretch flangeability of the steel sheet. Therefore, theTi content is 0.005% or more and 0.200% or less. The Ti content ispreferably 0.010% or more. The Ti content is preferably 0.100% or less.

The basic components according to this disclosure have been describedabove. The balance other than the components described above is Fe andinevitable impurities. Additionally, the following elements may b eoptionally contained as appropriate.

The chemical composition of the steel may further contain at least oneselected from the group consisting of Al: 0.01% or more and 2.00% orless, Nb: 0.005% or more and 0.200% or less, B: 0.0003% or more and0.0050% or less, Ni: 0.005% or more and 1.000% or less, Cr: 0.005% ormore and 1.000% or less, V: 0.005% or more and 0.500% or less, Mo:0.005% or more and 1.000% or less, Cu: 0.005% or more and 1.000% orless, Sn: 0.002% or more and 0.200% or less, Sb: 0.002% or more and0.200% or less, Ta: 0.001% or more and 0.010% or less, Ca: 0.0005% ormore and 0.0050% or less, Mg: 0.0005% or more and 0.0050% or less, andREM: 0.0005% or more and 0.0050% or less.

Al is a useful element for increasing the area of a ferrite-austenitedual phase region and reducing annealing temperature dependency, i.e.,increasing the stability of the steel sheet as a material. In addition,Al acts as a deoxidizer, and is also a useful element for maintainingthe cleanliness of the steel. If the Al content is below 0.01%, however,the addition effect is limited. Thus the lower limit is 0.01%. On theother hand, excessively adding Al beyond 2.00% increases the risk ofcracking occurring in a semi-finished product during continuous casting,and inhibits manufacturability. From these perspectives, the Al contentis 0.01% or more and 2.00% or less. The Al content is preferably 0.20%or more. The Al content is preferably 1.20% or less.

Nb is useful for achieving strengthening by precipitation of the steel.The addition effect can be obtained when the content is 0.005% or more.Nb can also ensure a desired area ratio of non-recrystallized ferrite,as in the case of adding Ti, and contributes to increasing the yieldratio of the steel sheet. Additionally, making use of relatively hardnon-recrystallized ferrite, Nb can reduce the difference in hardnessfrom a hard secondary phase (martensite or retained austenite), and alsocontributes to improving stretch flangeability. On the other hand, ifthe Nb content in the steel exceeds 0.200%, hard martensite excessivelyincreases in area ratio, which causes more microvoids at grainboundaries of martensite and facilitates propagation of cracks duringbend test and hole expansion test. This leads to a reduction in thebendability and stretch flangeability of the steel sheet. This alsoincreases cost. Therefore, when added to steel, the Nb content is 0.005%or more and 0.200% or less. The Nb content is preferably 0.010% or more.The Nb content is preferably 0.100% or less.

B may be added as necessary, since it has the effect of suppressing thegeneration and growth of ferrite from austenite grain boundaries andenables microstructure control according to the circumstances. Theaddition effect can be obtained when the B content is 0.0003% or more.If the B content exceeds 0.0050%, however, the formability of the steelsheet degrades. Therefore, when added to steel, the B content is 0.0003%or more and 0.0050% or less. The B content is preferably 0.0005% ormore. The B content is preferably 0.0030% or less.

Ni is an element that stabilizes retained austenite, and is thus auseful element for ensuring good ductility, and that increases thestrength of the steel through solid solution strengthening. The additioneffect can be obtained when the Ni content is 0.005% or more. On theother hand, if the Ni content in the steel exceeds 1.000%, hardmartensite excessively increases in area ratio, which causes moremicrovoids at grain boundaries of martensite and facilitates propagationof cracks during bend test and hole expansion test. This leads to areduction in the bendability and stretch flangeability of the steelsheet. This also increases cost. Therefore, when added to steel, the Nicontent is 0.005% or more and 1.000% or less.

Cr, V, and Mo are elements that may be added as necessary, since theyhave the effect of improving the balance between strength and ductility.The addition effect can be obtained when the Cr content is 0.005% ormore, the V content is 0.005% or more, and/or the Mo content is 0.005%or more. However, if the Cr content exceeds 1.000%, the V contentexceeds 0.500%, and/or the Mo content exceeds 1.000%, hard martensiteexcessively increases in area ratio, which causes more microvoids atgrain boundaries of martensite and facilitates propagation of cracksduring bend test and hole expansion test. This leads to a reduction inthe bendability and stretch flangeability of the steel sheet, and alsocauses a rise in cost. Therefore, when added to steel, the Cr content is0.005% or more and 1.000% or less, the V content is 0.005% or more and0.500% or less, and/or the Mo content is 0.005% or more and 1.000% orless.

Cu is a useful element for strengthening of steel and may be added forstrengthening of steel, as long as the content is within the rangedisclosed herein. The addition effect can be obtained when the Cucontent is 0.005% or more. On the other hand, if the Cu content in thesteel exceeds 1.000%, hard martensite excessively increases in arearatio, which causes more microvoids at grain boundaries of martensiteand facilitates propagation of cracks during bend test and holeexpansion test. This leads to a reduction in the bendability and stretchflangeability of the steel sheet. Therefore, when added to steel, the Cucontent is 0.005% or more and 1.000% or less.

Sn and Sb are elements that may be added as necessary from theperspective of suppressing decarbonization of a region extending fromthe surface layer of the steel sheet to a depth of about several tens ofmicrometers, which results from nitriding and/or oxidation of the steelsheet surface. Suppressing nitriding and/or oxidation in this way isuseful for preventing a reduction in the area ratio of martensite in thesteel sheet surface, and for ensuring the TS and stability of the steelsheet as a material. However, excessively adding Sn or Sb beyond 0.200%reduces toughness. Therefore, when Sn and/or Sb is added to steel, thecontent of each added element is 0.002% or more and 0.200% or less.

Ta forms alloy carbides or alloy carbonitrides, and contributes toincreasing the strength of the steel, as is the case with Ti and Nb. Itis also believed that Ta has the effect of effectively suppressingcoarsening of precipitates when partially dissolved in Nb carbides or Nbcarbonitrides to form complex precipitates, such as (Nb, Ta) (C, N), andproviding a stable contribution to increasing the strength of the steelsheet through strengthening by precipitation. Therefore, Ta ispreferably added to the steel according to the disclosure. The additioneffect of Ta can be obtained when the Ta content is 0.001% or more.Excessively adding Ta, however, fails to increase the addition effect,but instead results in a rise in alloying cost. Therefore, when added tosteel, the Ta content is 0.001% or more and 0.010% or less.

Ca, Mg, and REM are useful elements for causing spheroidization ofsulfides and mitigating the adverse effect of sulfides on hole expansionformability (stretch flangeability). To obtain this effect, it isnecessary to add any of these elements to steel in an amount of 0.0005%or more. However, if the content of each added element exceeds 0.0050%,more inclusions occur, for example, and some defects such as surfacedefects and internal defects are caused in the steel sheet. Therefore,when Ca, Mg, and/or REM is added to steel, the content of each addedelement is 0.0005% or more and 0.0050% or less.

The following provides a description of the microstructure. Sufficientductility of the steel sheet can be ensured by facilitating theformation of polygonal ferrite in the microstructure. This, however,causes decreases in tensile strength and yield strength. Besides, thetensile strength also varies depending on the area ratio of martensite,and the ductility is greatly affected by the amount of retainedaustenite. Hence, the mechanical properties of the high-strength steelsheet can be effectively obtained by controlling the amounts (arearatio, volume fraction) of these phases (microstructures). As a resultof conducting studies from this perspective, we newly discovered thatthe area ratios of polygonal ferrite and non-recrystallized ferrite arecontrollable by the rolling reduction in cold rolling. We also found outthat the area ratio of martensite and the volume fraction of retainedaustenite are mainly determined by the addition amount of Mn. We furtherfound out that, by omitting cold rolling or by limiting the rollingreduction in cold rolling to 30% or less, not only the area ratio ofpolygonal ferrite is reduced (i.e. can be controlled to an appropriaterange) (relative to the whole microstructure), but also themicrostructure shape of the final product changes greatly, yielding asteel sheet having crystal grains with a high aspect ratio. The value ofhole expansion formability λ is thus improved. In detail, themicrostructure of a steel sheet with high ductility and favorable holeexpansion formability is as follows.

Area Ratio of Polygonal Ferrite: 5% or More and 50% or Less

According to the disclosure, the area ratio of polygonal ferrite needsto be 5% or more to ensure sufficient ductility. On the other hand, toguarantee a strength of 980 MPa or more, the area ratio of softpolygonal ferrite needs to be 50% or less. The area ratio of polygonalferrite is preferably 10% or more. The area ratio of polygonal ferriteis preferably 40% or less. As used herein, “polygonal ferrite” refers toferrite that is relatively soft and that has high ductility.

Area Ratio of Non-Recrystallized Ferrite: 10% or More

In this disclosure, it is very important to set the area ratio ofnon-recrystallized ferrite to be 10% or more. In this regard,non-recrystallized ferrite is useful for increasing the strength of thesteel sheet. However, non-recrystallized ferrite may cause a significantdecrease in the ductility of the steel sheet, and thus is normallyreduced in a general process. In contrast, according to the presentdisclosure, by using polygonal ferrite and retained austenite to providegood ductility and intentionally utilizing relatively hardnon-recrystallized ferrite, it is possible to provide the steel sheetwith the intended TS, without having to form a large amount ofmartensite, such as exceeding 30% in area ratio.

Moreover, according to the present disclosure, interfaces betweendifferent phases, namely, between polygonal ferrite and martensite, arereduced, making it possible to increase the yield point (YP) and YR ofthe steel sheet.

To obtain these effects, the area ratio of non-recrystallized ferriteneeds to be 10% or more. The area ratio of non-recrystallized ferrite ispreferably 13% or more.

As used herein, “non-recrystallized ferrite” refers to ferrite thatcontains strain in the grains with a crystal orientation difference ofless than 15°, and that is harder than the above-described polygonalferrite with high ductility.

In the disclosure, no upper limit is placed on the area ratio ofnon-recrystallized ferrite, yet a preferred upper limit is around 45%,considering the possibility of increased material anisotropy in thesteel sheet surface.

Area Ratio of Martensite: 15% or More and 30% or Less

To achieve TS of 980 MPa or more, the area ratio of martensite needs tobe 15% or more. On the other hand, to ensure good ductility, the arearatio of martensite needs to be limited to 30% or less.

According to the disclosure, the area ratios of ferrite (includingpolygonal ferrite and non-recrystallized ferrite) and martensite can bedetermined in the following way.

Specifically, a cross section of a steel sheet that is taken in thesheet thickness direction to be parallel to the rolling direction (whichis an L-cross section) is polished, then etched with 3 vol. % nital, andten locations are observed at 2000 times magnification under an SEM(scanning electron microscope), at a position of sheet thickness×¼(which is the position at a depth of one-fourth of the sheet thicknessfrom the steel sheet surface), to capture microstructure micrographs.The captured microstructure micrographs are used to calculate the arearatios of respective phases (ferrite and martensite) for the tenlocations using Image-Pro manufactured by Media Cybernetics, the resultsare averaged, and each average is used as the area ratio of thecorresponding phase. In the microstructure micrographs, polygonalferrite and non-recrystallized ferrite appear as a gray structure (basesteel structure), while martensite as a white structure.

According to the disclosure, the area ratios of polygonal ferrite andnon-recrystallized ferrite can be determined in the following way.Specifically, low-angle grain boundaries in which the crystalorientation difference is from 2° to less than 15° and large-angle grainboundaries in which the crystal orientation difference is 15° or moreare identified using EBSD (Electron Backscatter Diffraction). An IQ Mapis then created, considering ferrite that contains low-angle grainboundaries in the grains as non-recrystallized ferrite. Then, low-anglegrain boundaries and large-angle grain boundaries are extracted from thecreated IQ Map at ten locations, respectively, to determine the areas oflow-angle grain boundaries and large-angle grain boundaries at the tenlocations. Based on the results, the areas of polygonal ferrite andnon-recrystallized ferrite are calculated to determine the area ratiosof polygonal ferrite and non-recrystallized ferrite for the tenlocations. By averaging the results, the above-described area ratios ofpolygonal ferrite and non-recrystallized ferrite are determined.

Volume Fraction of Retained Austenite: 12% or More

According to the disclosure, the volume fraction of retained austeniteneeds to be 12% or more, to ensure sufficient ductility. The volumefraction of retained austenite is preferably 14% or more.

According to the disclosure, no upper limit is placed on the area ratioof retained austenite, yet a preferred upper limit is around 50%,considering the risk of formation of increased amounts of unstableretained austenite resulting from insufficient concentration of C, Mn,and the like, which is less effective in improving ductility.

The volume fraction of retained austenite is calculated by determiningthe x-ray diffraction intensity of a plane of sheet thickness×¼ (whichis the plane at a depth of one-fourth of the sheet thickness from thesteel sheet surface), which is exposed by polishing the steel sheetsurface to a depth of one-fourth of the sheet thickness. Using anincident x-ray beam of MoKα, the intensity ratio of the peak integratedintensity of the {111}, {200}, {220}, and {311} planes of retainedaustenite to the peak integrated intensity of the {110}, {200}, and{211} planes of ferrite is calculated for all of the twelvecombinations, the results are averaged, and the average is used as thevolume fraction of retained austenite.

Average Grain Size of Polygonal Ferrite: 4 μm or Less

Refinement of polygonal ferrite grains contributes to improving YP andTS. Thus, to ensure a high YP and a high YR as well as a desired TS,polygonal ferrite needs to have an average grain size of 4 μm or less.The average grain size of polygonal ferrite is preferably 3 μm or less.

According to the disclosure, no lower limit is placed on the averagegrain size of polygonal ferrite, yet, from an industrial perspective, apreferred lower limit is around 0.2 μm.

Average Grain Size of Martensite: 2 μm or Less

Refinement of martensite grains contributes to improving bendability andstretch flangeability (hole expansion formability). Thus, to ensure highbendability and high stretch flangeability (high hole expansionformability), the average grain size of martensite needs to be limitedto 2 μm or less. The average grain size of martensite is preferably 1.5μm or less.

According to the disclosure, no lower limit is placed on the averagegrain size of martensite, yet, from an industrial perspective, apreferred lower limit is around 0.05 μm.

Average Grain Size of Retained Austenite: 2 μm or Less

Refinement of retained austenite grains contributes to improvingductility, as well as bendability and stretch flangeability (holeexpansion formability). Accordingly, to ensure good ductility,bendability, and stretch flangeability (hole expansion formability) ofthe steel sheet, the average grain size of retained austenite needs tobe 2 μm or less. The average grain size of retained austenite ispreferably 1.5 μm or less.

According to the disclosure, no lower limit is placed on the averagegrain size of retained austenite, yet, from an industrial perspective, apreferred lower limit is around 0.05 μm.

The average grain sizes of polygonal ferrite, martensite, and retainedaustenite are respectively determined by averaging the results fromcalculating equivalent circular diameters from the areas of polygonalferrite grains, martensite grains, and retained austenite grainsmeasured with Image-Pro as mentioned above. Polygonal ferrite,non-recrystallized ferrite, martensite, and retained austenite areseparated using EBSD, and martensite and retained austenite areidentified using an EBSD phase map. In this case, each of theabove-described average grain sizes is determined from the measurementsfor grains with a grain size of 0.01 μm or more. The reason is thatgrains with a grain size of less than 0.01 μm have no effect on thedisclosure.

Average Aspect Ratio of Crystal Grains of Each of Polygonal Ferrite,Martensite, and Retained Austenite: 2.0 or More and 20.0 or Less

In this disclosure, it is very important to set the average aspect ratioof crystal grains of each of polygonal ferrite, martensite, and retainedaustenite to 2.0 or more.

A lower aspect ratio of crystal grains indicates that, during retentionin heat treatment after cold rolling (cold-rolled sheet annealing),ferrite and austenite recover and recrystallize and then undergo graingrowth, resulting in the formation of crystal grains close to equiaxedgrains. The ferrite formed here is soft. In the case where cold rollingis omitted or the rolling reduction in cold rolling is less than 30%, onthe other hand, the amount of strain applied decreases, so that theformation of polygonal ferrite is suppressed and a microstructure mainlycomposed of crystal grains with a high aspect ratio results. Such amicrostructure composed of crystal grains with a high aspect ratio ishard because it contains a large amount of strain or has parts where thedistance between grain boundaries is short, as compared with theabove-mentioned microstructure. Therefore, not only the TS is improved,but also the difference in hardness from hard phases such as retainedaustenite and martensite decreases, and the hole expansion formabilityis improved without loss of ductility. If the aspect ratio is more than20.0, the TS increases extremely, and favorable ductility cannot beachieved.

Thus, the average aspect ratio of crystal grains of each of polygonalferrite, martensite, and retained austenite is limited to 2.0 or moreand 20.0 or less. In terms of improving ductility, the average aspectratio is more preferably 2.2 or more, and more preferably 2.4 or more.

The aspect ratio of a crystal grain mentioned here is a value obtainedby dividing the major axis length of the crystal grain by the minor axislength of the crystal grain. The average aspect ratio of each type ofcrystal grains can be calculated as follows.

For each of polygonal ferrite grains, martensite grains, and retainedaustenite grains, the major axis length and minor axis length of each of30 crystal grains are calculated using the above-mentioned Image-Pro,the major axis length is divided by the minor axis length, and thedivision results are averaged.

A Value Obtained by Dividing the Mn Content in the Retained Austenite(in Mass %) by the Mn Content in the Polygonal Ferrite (in Mass %): 2.0or More

In this disclosure, it is very important that the value obtained bydividing the Mn content in the retained austenite (in mass %) by the Mncontent in the polygonal ferrite (in mass %) equals 2.0 or more. Thereason is that better ductility requires a larger amount of stableretained austenite with concentrated Mn.

According to the disclosure, no upper limit is placed on the valueobtained by dividing the Mn content in the retained austenite (in mass%) by the Mn content in the polygonal ferrite (in mass %), yet apreferred upper limit is around 16.0 from the perspective of ensuringstretch flangeability.

The Mn content in the retained austenite (in mass %) and the Mn contentin the polygonal ferrite (in mass %) can be determined in the followingway.

Specifically, an EPMA (Electron Probe Micro Analyzer) is used toquantify the distribution of Mn in each phase in a cross section alongthe rolling direction at a position of sheet thickness×¼. Then, 30retained austenite grains and 30 ferrite grains are analyzed todetermine Mn contents, the results are averaged, and each average isused as the Mn content in the corresponding phase.

In addition to the above-described polygonal ferrite, martensite, and soon, the microstructure according to the disclosure may further includecarbides ordinarily found in steel sheets, such as granular ferrite,acicular ferrite, bainitic ferrite, tempered martensite, pearlite, andcementite (excluding cementite in pearlite). Any of these structures maybe included as long as the area ratio is 10% or less, without impairingthe effect of the disclosure.

According to the disclosure, the steel microstructure preferablycontains, in area ratio, 2% or more of ε phase with an hcp (hexagonalclosest packing) structure. In this respect, steel may become brittlewhen it contains a large amount of ε phase with an hcp structure. As inthe present disclosure, however, when an appropriate amount of ε phasewith an hcp structure is finely distributed within and along boundariesof polygonal ferrite and non-recrystallized ferrite grains, it becomespossible to achieve excellent damping performance, while keeping a goodbalance between strength and ductility.

Such ε phase with an hcp structure, martensite, and retained austenitecan be identified using an EBSD phase map. In this disclosure, no upperlimit is placed on the area ratio of ε phase, yet, in view of the riskof embrittlement of the steel, a preferred upper limit is around 35%.

We made further investigations on the microstructures of steel sheetsupon performing press forming and working.

As a result, it was discovered that there are two types of retainedaustenite: one transforms to martensite immediately upon the subjectionof the steel sheet to press forming or working, while the other persistsuntil the working ratio becomes high enough to cause the retainedaustenite to eventually transform to martensite, bringing about a TRIPphenomenon (transformation induced plasticity phenomenon). It was alsorevealed that good elongation can be obtained in a particularlyeffective way when a large amount of retained austenite transforms tomartensite after the working ratio becomes high enough.

Specifically, as a result of collecting samples with good and poorelongation and measuring the quantity of retained austenite by varyingthe degree of tensile working from 0% to 20%, the working ratio and thequantity of retained austenite showed a tendency as illustrated inFIG. 1. As used herein, “the working ratio” refers to the elongationratio that is determined from a tensile test performed on a JIS No. 5test piece sampled from a steel sheet with the tensile direction beingperpendicular to the rolling direction of the steel sheet.

It can be seen from FIG. 1 that the samples with good elongation eachshowed a gentle decrease in the quantity of retained austenite as theworking ratio increased.

Accordingly, we further measured the quantity of retained austenite ineach sample with TS 980 MPa grade after subjection to tensile workingwith an elongation value of 10%, and examined the effect of the ratio ofthe quantity of retained austenite after the tensile working to thequantity before the tensile working on the total elongation of the steelsheet. The results are shown in FIG. 2.

It can be seen from FIG. 2 that elongation is good if the value obtainedby dividing the volume fraction of retained austenite remaining in asteel after subjection to tensile working with an elongation value of10% by the volume fraction of retained austenite before the tensileworking equals 0.3 or more, but otherwise elongation is poor.

Therefore, it is preferable in the disclosure that the value obtained bydividing the volume fraction of retained austenite remaining in a steelafter subjection to tensile working with an elongation value of 10% bythe volume fraction of retained austenite before the tensile workingequals 0.3 or more. The reason is that this set up may ensure thetransformation of sufficient retained austenite to martensite after theworking ratio becomes high enough.

The above-described TRIP phenomenon requires retained austenite to bepresent before performing press forming or working. To cause retainedaustenite to be present before performing press forming or working, theMs point (martensite transformation start temperature) which depends onthe elements contained in the steel microstructure needs to be as low asapproximately 15° C. or lower.

Specifically, in the tensile working with an elongation value of 10%according to the disclosure, a tensile test is performed on a JIS No. 5test piece sampled from a steel sheet with the tensile direction beingperpendicular to the rolling direction of the steel sheet, and the testis interrupted when the elongation ratio reaches 10%, thus applyingtensile working with an elongation value of 10% to the test piece.

The volume fraction of retained austenite can be determined in theabove-described way.

Upon a detailed study of samples satisfying the above conditions, wediscovered that a TRIP phenomenon providing high strain hardenabilityoccurs upon working and even better elongation can be achieved if the Ccontent and the Mn content in the retained austenite satisfy thefollowing relation:

0.04*[Mn]+0.056−0.180≤[C]≤0.04*[Mn]+0.056+0.180

where

[C] is the C content in the retained austenite in mass %, and

[Mn] is the Mn content in the retained austenite in mass %.

When the above requirements are met, it is possible to cause a TRIPphenomenon, which is a key factor of improving ductility, to occurintermittently up until the final stage of working performed on thesteel sheet, guaranteeing the generation of so-called stable retainedaustenite.

The C content in the retained austenite (in mass %) can be determined inthe following way.

Specifically, an EPMA is used to quantify the distribution of C in eachphase in a cross section along the rolling direction at a position ofsheet thickness×¼. Then, 30 retained austenite grains are analyzed todetermine C contents, the results are averaged, and the average is usedas the C content. Note that the Mn content in the retained austenite (inmass %) can be determined in the same way as the C content in theretained austenite.

The following describes the production conditions.

Steel Slab Heating Temperature: 1100° C. or Higher and 1300° C. or Lower

Precipitates that are present at the time of heating of a steel slab(hereinafter, also referred to simply as a “slab”) will remain as coarseprecipitates in the resulting steel sheet, making no contribution tostrength. Thus, remelting of any Ti- and Nb-based precipitates formedduring casting is required.

In this respect, if a steel slab is heated at a temperature below 1100°C., it is difficult to cause sufficient dissolution of carbides, leadingto problems such as an increased risk of trouble during the hot rollingresulting from increased rolling load. Therefore, the steel slab heatingtemperature is preferably 1100° C. or higher.

In addition, from the perspective of obtaining a smooth steel sheetsurface by scaling-off defects in the surface layer of the slab, such asblow hole generation, segregation, and the like, and reducing cracks andirregularities over the steel sheet surface, the steel slab heatingtemperature is preferably 1100° C. or higher.

If the steel slab heating temperature exceeds 1300° C., however, scaleloss increases as oxidation progresses. Therefore, the steel slabheating temperature is preferably 1300° C. or lower. For this reason,the steel slab heating temperature is preferably 1100° C. or higher and1300° C. or lower. The steel slab heating temperature is furtherpreferably 1150° C. or higher. The steel slab heating temperature isfurther preferably 1250° C. or lower.

A steel slab is preferably made with continuous casting to prevent macrosegregation, yet may be produced with other methods such as ingotcasting or thin slab casting. The steel slab thus produced may be cooledto room temperature and then heated again according to a conventionalprocess. Moreover, energy-saving processes are applicable without anyproblem, such as hot direct rolling or direct rolling in which either awarm steel slab without being fully cooled to room temperature ischarged into a heating furnace, or a steel slab is hot rolledimmediately after being subjected to heat retaining for a short period.A steel slab is subjected to rough rolling under normal conditions andformed into a sheet bar. When the heating temperature is low, it ispreferable to additionally heat the sheet bar using a bar heater or thelike prior to finish rolling, from the viewpoint of preventing troublesduring the hot rolling.

Finisher Delivery Temperature in Hot Rolling: 750° C. or Higher and1000° C. or Lower

The heated steel slab is hot rolled through rough rolling and finishrolling to form a hot-rolled sheet. At this point, when the finisherdelivery temperature exceeds 1000° C., the amount of oxides (scales)generated suddenly increases and the interface between the steelsubstrate and oxides becomes rough, which tends to lower the surfacequality of the steel sheet after subjection to pickling and coldrolling. In addition, any hot rolling scales persisting after picklingadversely affect the ductility and stretch flangeability of the steelsheet. Moreover, grain size is excessively coarsened, causing surfacedeterioration in a pressed part during working. On the other hand, ifthe finisher delivery temperature is below 750° C., rolling loadincreases and rolling is performed more often with austenite being in anon-recrystallized state. As a result, an abnormal texture develops inthe steel sheet, and the final product has a significant planaranisotropy such that the material properties not only become lessuniform (the stability as a material decreases), but the ductilityitself also deteriorates. Besides, if the finisher delivery temperaturein the hot rolling is lower than 750° C. or higher than 1000° C., amicrostructure having 15% or more and 30% or less of martensite in arearatio and 12% or more of retained austenite in volume fraction cannot beobtained.

Therefore, the finisher delivery temperature in the hot rolling needs tobe 750° C. or higher and 1000° C. or lower. The finisher deliverytemperature is preferably 800° C. or higher. The finisher deliverytemperature is preferably 950° C. or lower.

Average Coiling Temperature after Hot Rolling: 300° C. or Higher and750° C. or Lower

When the average coiling temperature after the hot rolling is above 750°C., the grain size of ferrite in the microstructure of the hot-rolledsheet increases, making it difficult to ensure a desired strength of thefinal-annealed sheet. On the other hand, when the average coilingtemperature after the hot rolling is below 300° C., there is an increasein the strength of the hot-rolled sheet and in the rolling load for coldrolling, and the steel sheet suffers malformation. As a result,productivity decreases. Therefore, the average coiling temperature afterthe hot rolling is preferably 300° C. or higher and 750° C. or lower.The average coiling temperature is more preferably 400° C. or higher.The average coiling temperature is more preferably 650° C. or lower.

According to the disclosure, finish rolling may be performedcontinuously by joining rough-rolled sheets during the hot rolling.Rough-rolled sheets may be coiled on a temporary basis. At least part offinish rolling may be conducted as lubrication rolling to reduce therolling load during the hot rolling. Conducting lubrication rolling insuch a manner is effective from the perspective of making the shape andmaterial properties of the steel sheet uniform. In lubrication rolling,the coefficient of friction is preferably 0.10 or more. The coefficientof friction is preferably 0.25 or less.

The hot-rolled sheet thus produced is subjected to pickling. Picklingenables removal of oxides from the steel sheet surface, and is thusimportant to ensure that the high-strength steel sheet as the finalproduct has good chemical convertibility and sufficient coating quality.The pickling may be performed in one or more batches.

Hot Band Annealing (First Heat Treatment): To Retain in a TemperatureRange of [Ac₁ Transformation Temperature+20° C.] to [Ac₁ TransformationTemperature+120° C.] for 600 s to 21,600 s

In this disclosure, it is very important to retain the steel sheet in atemperature range of [Ac₁ transformation temperature+20° C.] to [Ac₁transformation temperature+120° C.] for 600 s to 21,600 s.

If the hot band annealing is performed at an annealing temperature below[Ac₁ transformation temperature+20° C.] or above [Ac₁ transformationtemperature+120° C.], or if the holding time is shorter than 600 s,concentration of Mn in austenite does not proceed in either case, makingit difficult to ensure a sufficient volume fraction of retainedaustenite after the final annealing. As a result, ductility decreases.Besides, a microstructure in which the value obtained by dividing the Mncontent in retained austenite (in mass %) by the Mn content in polygonalferrite (in mass %) equals 2.0 or more cannot be obtained. On the otherhand, if the steel sheet is retained for more than 21,600 s,concentration of Mn in austenite reaches a plateau, and becomes lesseffective in improving ductility after the final annealing, resulting ina rise in costs.

Therefore, in the hot band annealing (first heat treatment) according tothe disclosure, the steel sheet is retained in a temperature range of[Ac₁ transformation temperature+20° C.] to [Ac₁ transformationtemperature+120° C.] for 600 s to 21,600 s.

The above-described heat treatment process may be continuous annealingor batch annealing. After the above-described heat treatment, the steelsheet is cooled to room temperature. The cooling process and coolingrate are not particularly limited, however, and any type of cooling maybe performed, including furnace cooling and air cooling in batchannealing and gas jet cooling, mist cooling, and water cooling incontinuous annealing. The pickling may be performed according to aconventional process.

Annealing (Second Heat Treatment): To Retain in a Temperature Range of[Ac₁ Transformation Temperature+10° C.] to [Ac₁ TransformationTemperature+100° C.] for 20 s to 900 s

In this disclosure, it is very important to retain the steel sheet in atemperature range of [Ac₁ transformation temperature+10° C.] to [Ac₁transformation temperature+100° C.] for 20 s to 900 s. When theannealing temperature is below [Ac₁ transformation temperature+10° C.]or above [Ac₁ transformation temperature+100° C.], or if the holdingtime is shorter than 20 s, concentration of Mn in austenite does notproceed in either case, making it difficult to ensure a sufficientvolume fraction of retained austenite. As a result, ductility decreases.Besides, a microstructure in which the average grain size of polygonalferrite is 4 μm or less, the average grain size of martensite is 2 μm orless, the average grain size of retained austenite is 2 or less, and thevalue obtained by dividing the Mn content in retained austenite (in mass%) by the Mn content in polygonal ferrite (in mass %) equals 2.0 or morecannot be obtained. On the other hand, if the steel sheet is retainedfor more than 900 s, the area ratio of non-crystallized ferritedecreases and the interfaces between different phases, namely, betweenferrite and hard secondary phases (martensite and retained austenite),are reduced, leading to a reduction in both YP and YR. Besides, amicrostructure in which the area ratio of non-recrystallized ferrite is10% or more, a microstructure in which the average grain size ofmartensite is 2 μm or less and the average grain size of retainedaustenite is 2 μm or less, and a microstructure in which the valueobtained by dividing the Mn content in retained austenite (in mass %) bythe Mn content in polygonal ferrite (in mass %) equals 2.0 or morecannot be obtained.

Rolling Reduction in Cold Rolling: Less than 30%

Cold rolling may be performed after the hot band annealing and beforethe annealing (second heat treatment). In this case, the rollingreduction needs to be less than 30%. By omitting the cold rolling orperforming the cold rolling with a rolling reduction of less than 30%,polygonal ferrite which forms by recrystallization after the heattreatment does not form and a microstructure elongated in the rollingdirection remains, and eventually polygonal ferrite, retained austenite,and martensite with a high aspect ratio are obtained. Thus, not only thestrength-ductility balance is improved, but also the stretchflangeability (hole expansion formability) is improved. If the rollingreduction is 30% or more, a microstructure having an average aspectratio of crystal grains of each of martensite and retained austenite of2.0 or more and 20.0 or less cannot be obtained.

Hot-Dip Galvanizing Treatment

In hot-dip galvanizing treatment according to the disclosure, the steelsheet subjected to the above-described annealing (second heat treatment)is dipped in a galvanizing bath at 440° C. or higher and 500° C. orlower for hot-dip galvanizing. Subsequently, the coating weight on thesteel sheet surface is adjusted using gas wiping or the like.Preferably, the hot-dip galvanizing is performed using a galvanizingbath containing 0.10 mass % or more and 0.22 mass % or less of Al.

Moreover, when a hot-dip galvanized layer is subjected to alloyingtreatment, the alloying treatment may be performed in a temperaturerange of 450° C. to 600° C. after the above-described hot-dipgalvanizing treatment. If the alloying treatment is performed at atemperature above 600° C., untransformed austenite transforms topearlite, where a desired volume fraction of retained austenite cannotbe ensured and ductility degrades. On the other hand, if the alloyingtreatment is performed at a temperature below 450° C., the alloyingprocess does not proceed, making it difficult to form an alloy layer.

Therefore, when the galvanized layer is subjected to alloying treatment,the alloying treatment is performed in a temperature range of 450° C. to600° C.

Although other manufacturing conditions are not particularly limited,the series of processes including the annealing, hot-dip galvanizing,and alloying treatment described above may preferably be performed in acontinuous galvanizing line (CGL), which is a hot-dip galvanizing line,from the perspective of productivity.

When hot-dip aluminum coating treatment is performed, the steel sheetsubjected to the above-described annealing treatment is dipped in analuminum molten bath at 660° C. to 730° C. for hot-dip aluminum coatingtreatment. Subsequently, the coating weight is adjusted using gas wipingor the like. If the steel sheet has a composition such that thetemperature of the aluminum molten bath falls within the temperaturerange of [Ac₁ transformation temperature+10° C.] to [Ac₁ transformationtemperature+100° C.], the steel sheet is preferably subjected to hot-dipaluminum coating treatment because finer and more stable retainedaustenite can be formed, and therefore further improvement in ductilitycan be achieved.

Electrogalvanizing Treatment

According to the disclosure, electrogalvanizing treatment may also beperformed on the steel sheet after the heat treatment. No particularlimitations are placed on the electrogalvanizing treatment conditions,yet the electrogalvanizing treatment conditions are preferably set sothat the plated layer has a thickness of 5 μm to 15 μm.

According to the disclosure, the above-described steel sheet, hot-dipgalvanized steel sheet, hot-dip aluminum-coated steel sheet, andelectrogalvanized steel sheet may be subjected to skin pass rolling forthe purposes of straightening, adjustment of roughness on the sheetsurface, and the like. The skin pass rolling is preferably performed ata rolling reduction of 0.1% or more. The skin pass rolling is preferablyperformed at a rolling reduction of 2.0% or less.

When the rolling reduction is less than 0.1%, the skin pass rollingbecomes less effective and more difficult to control. Thus, a preferablerange for the rolling reduction has a lower limit of 0.1%. On the otherhand, when the skin pass rolling is performed at a rolling reductionabove 2.0%, the productivity of the steel sheet decreases significantly.Thus, the preferable range for the rolling reduction has an upper limitof 2.0%.

The skin pass rolling may be performed on-line or off-line. Skin passmay be performed in one or more batches to achieve a target rollingreduction.

Moreover, the steel sheet, the hot-dip galvanized steel sheet, thehot-dip aluminum-coated steel sheet, and the electrogalvanized steelsheet according to the disclosure may be subjected to a variety ofcoating treatment options, such as those using coating of resin, fatsand oils, and the like.

Examples

Steels having the chemical compositions as presented in Table 1, withthe balance consisting of Fe and inevitable impurities, were prepared bysteelmaking in a converter, and formed into slabs through continuouscasting. The slabs thus obtained were formed into a variety of steelsheets, as described below, by varying the conditions as listed in Table2.

After being hot rolled, each steel sheet was annealed in a temperaturerange of [Ac₁ transformation temperature+20° C.] to [Ac₁ transformationtemperature+120° C.]. After being cold rolled (or without cold rolling),each steel sheet was annealed in a temperature range of [Ac₁transformation temperature+10° C.] to [Ac₁ transformationtemperature+100° C.]. Consequently, a cold-rolled steel sheet (CR) wasobtained, and subjected to coating treatment to form a hot-dipgalvanized steel sheet (GI), a galvannealed steel sheet (GA), a hot-dipaluminum-coated steel sheet (Al), an electrogalvanized steel sheet (EG),or the like.

Used as hot-dip galvanizing baths were a zinc bath containing 0.19 mass% of Al for hot-dip galvanized steel sheets (GI) and a zinc bathcontaining 0.14 mass % of Al for galvannealed steel sheets (GA). Ineither case, the bath temperature was 465° C. and the coating weight perside was 45 g/m² (in the case of both-sided coating). For GA, the Feconcentration in the coating layer was adjusted to be 9 mass % or moreand 12 mass % or less. The bath temperature of the hot-dip aluminummolten bath for hot-dip aluminum-coated steel sheets was set at 700° C.

For each of the steel sheets thus obtained, the cross-sectionalmicrostructure, tensile property, hole expansion formability,bendability, and the like were investigated. The results are listed inTables 3 to 5.

The Ac₁ transformation temperature was calculated by:

[Ac₁ transformation temperature (° C.)]=751−16*(% C)+11*(% Si)−28*(%Mn)−5.5*(% Cu)−16*(% Ni)+13*(% Cr)+3.4*(% Mo)

where (% C), (% Si), (% Mn), (% Ni), (% Cu), (% Cr), and (% Mo) eachrepresent the content in steel (in mass %) of the element in theparentheses.

Tensile test was performed in accordance with JIS Z 2241 (2011) tomeasure YP, YR, TS, and EL using JIS No. 5 test pieces, each of whichwas sampled in a manner that the tensile direction was perpendicular tothe rolling direction of the steel sheet. Note that YR is YP divided byTS, expressed as a percentage. In this case, the results were determinedto be good when YR≥68% and when TS*EL≥22,000 MPa %. Also, EL wasdetermined to be good when EL≥26% for TS 980 MPa grade, EL≥22% for TS1180 MPa grade, and EL≥18% for TS 1470 MPa grade. In this case, a steelsheet of TS 980 MPa grade refers to a steel sheet with TS of 980 MPa ormore and less than 1180 MPa, a steel sheet of TS 1180 MPa grade refersto a steel sheet with TS of 1180 MPa or more and less than 1470 MPa, anda steel sheet of TS 1470 MPa grade refers to a steel sheet with TS of1470 MPa or more and less than 1760 MPa.

Bend test was performed according to the V-block method specified in JISZ 2248 (1996). Each steel sheet was visually observed under astereoscopic microscope for cracks on the outside of the bent portion,and the minimum bending radius without cracks was used as the limitbending radius R. In this case, the bendability of the steel sheet wasdetermined to be good if the following condition was satisfied: limitbending radius R at 90° V-bending/t≤2.0 (where t is the thickness of thesteel sheet).

Hole expansion test was performed in accordance with JIS Z 2256 (2010).Each of the steel sheets obtained was cut to a size of 100 mm*100 mm,and a hole of 10 mm in diameter was drilled through each sample withclearance 12%±1%. Then, each steel sheet was clamped into a die havingan inner diameter of 75 mm with a blank holding force of 9 tons (88.26kN). In this state, a conical punch of 60° was pushed into the hole, andthe hole diameter at the crack initiation limit was measured. Then, toevaluate hole expansion formability, the maximum hole expansion ratio(%) was calculated by:

Maximum hole expansion ratio λ(%)={(D _(f) −D ₀)/D ₀}*100

where D_(f) is a hole diameter at the time of occurrence of cracking(mm) and D₀ is an initial hole diameter (mm).

In this case, the maximum hole expansion ratio was determined to be goodwhen λ≥25% for TS 980 MPa grade, λ≥18% for TS 1180 MPa grade, and λ≥15%for TS 1470 MPa grade.

The sheet passage ability during hot rolling was determined to be lowwhen it was considered that the risk of troubles, such as malformationduring hot rolling due to increased rolling load, would increasebecause, for example, the hot-rolling finisher delivery temperature waslow and rolling would be performed more often with austenite being in anon-crystallized state, or rolling would be performed in anaustenite-ferrite dual phase region. The sheet passage ability duringcold rolling was determined to be low when it was considered that therisk of troubles, such as malformation during cold rolling due toincreased rolling load, would increase because, for example, the coilingtemperature during hot rolling was low and the hot-rolled sheet had asteel microstructure in which low-temperature transformation phases,such as bainite and martensite, were dominantly present.

The surface characteristics of each final-annealed sheet were determinedto be poor when defects such as blow hole generation and segregation onthe surface layer of the slab could not be scaled-off, cracks andirregularities on the steel sheet surface increased, and a smooth steelsheet surface could not be obtained. The surface characteristics of eachfinal-annealed sheet were also determined to be poor when the amount ofoxides (scales) generated suddenly increased, interfaces between thesteel substrate and oxides were roughened, and the surface quality afterpickling and cold rolling degraded, or when hot-rolling scales persistedat least in part after pickling.

In this case, productivity was evaluated according to the lead timecosts, including: (1) malformation of a hot-rolled sheet occurred; (2) ahot-rolled sheet requires straightening before proceeding to thesubsequent steps; and (3) a prolonged holding time during the annealingtreatment. The productivity was determined to be “good” when none of (1)to (3) applied and “poor” when any of (1) to (3) applied.

Tensile working was performed in accordance with JIS Z 2241 (2011) usingJIS No. 5 test pieces, each of which was sampled in a manner that thetensile direction was perpendicular to the rolling direction of thesteel sheet. A value was obtained by dividing the volume fraction ofretained austenite remaining in each steel sheet after subjection totensile working with an elongation value of 10% by the volume fractionof retained austenite before the working (10% application). The volumefraction of retained austenite was measured in accordance with the aboveprocedure.

The measurement results are also listed in Table 4.

The C content in the retained austenite (in mass %) and the Mn contentin the retained austenite (in mass %) were measured in accordance withthe above procedure.

The measurement results are also listed in Table 4.

TABLE 1 Steel Chemical composition (mass %) temperature sample ID C SiMn P S N Ti Al Nb B Ni Cr V Mo Cu Sn Sb Ta Ca Mg REM (° C.) Remarks A0.121 0.50 5.03 0.022 0.0021 0.0037 0.025 — — — — — — — — — — — — — —613 Conforming steel B 0.178 0.50 5.55 0.023 0.0022 0.0039 0.037 — — — —— — — — — — — — — — 598 Conforming steel C 0.199 0.90 5.89 0.022 0.00220.0036 0.036 — — — — — — — — — — — — — — 593 Conforming steel D 0.1751.50 5.22 0.021 0.0024 0.0043 0.038 — — — — — — — — — — — — — — 619Conforming steel E 0.101 0.87 5.08 0.026 0.0021 0.0044 0.033 — — — — — —— — — — — — — — 617 Conforming steel F 0.167 0.10 5.01 0.024 0.00280.0033 0.023 — — — — — — — — — — — — — — 609 Conforming steel G 0.1150.99 5.23 0.026 0.0021 0.0036 0.031 — — — — — — — — — — — — — — 614Conforming steel H 0.120 0.75 4.77 0.027 0.0021 0.0036 0.025 — — — — — —— — — — — — — — 624 Conforming steel I 0.121 0.55 5.25 0.028 0.00280.0035 0.039 — — — — — — — — — — — — — — 608 Conforming steel IA 0.0300.55 5.12 0.025 0.0021 0.0031 0.041 — — — — — — — — — — — — — — 613Conforming steel IB 0.250 0.51 4.85 0.028 0.0022 0.0035 0.042 — — — — —— — — — — — — — — 617 Conforming steel IC 0.120 0.01 4.78 0.028 0.00210.0032 0.038 — — — — — — — — — — — — — — 615 Conforming steel ID 0.1213.00 5.01 0.031 0.0023 0.0035 0.036 — — — — — — — — — — — — — — 642Conforming steel IE 0.131 0.62 4.25 0.030 0.0021 0.0036 0.045 — — — — —— — — — — — — — — 637 Conforming steel IF 0.128 0.45 6.00 0.030 0.00280.0034 0.055 — — — — — — — — — — — — — — 586 Conforming steel IG 0.1210.51 4.89 0.001 0.0030 0.0040 0.045 — — — — — — — — — — — — — — 618Conforming steel IH 0.125 0.52 5.55 0.100 0.0031 0.0041 0.051 — — — — —— — — — — — — — — 599 Conforming steel II 0.142 0.55 4.99 0.028 0.00010.0038 0.054 — — — — — — — — — — — — — — 615 Conforming steel IJ 0.1530.48 4.89 0.025 0.0200 0.0032 0.051 — — — — — — — — — — — — — — 617Conforming steel IK 0.120 0.63 5.11 0.022 0.0031 0.0005 0.053 — — — — —— — — — — — — — — 613 Conforming steel IL 0.124 0.67 5.23 0.022 0.00350.0100 0.045 — — — — — — — — — — — — — — 610 Conforming steel IM 0.1420.51 4.99 0.021 0.0032 0.0038 0.005 — — — — — — — — — — — — — — 615Conforming steel IN 0.133 0.68 5.45 0.023 0.0036 0.0035 0.200 — — — — —— — — — — — — — — 604 Conforming steel IO 0.090 0.44 4.10 0.028 0.00210.0037 0.065 — — — — — — — — — — — — — — 640 Comparative steel IP 0.1450.61 5.03 0.030 0.0031 0.0034 0.210 — — — — — — — — — — — — — — 615Comparative steel J 0.022 0.67 5.71 0.021 0.0027 0.0034 0.041 — — — — —— — — — — — — — — 598 Comparative steel K 0.201 4.33 5.29 0.026 0.00260.0036 0.031 — — — — — — — — — — — — — — 647 Comparative steel L 0.2110.88 2.09 0.022 0.0020 0.0037 0.032 — — — — — — — — — — — — — — 699Comparative steel M 0.201 0.71 5.62 0.024 0.0037 0.0037 0.003 — — — — —— — — — — — — — — 598 Comparative steel N 0.205 0.51 5.55 0.020 0.00300.0040 0.038 0.35 — — — — — — — — — — — — — 598 Conforming steel O 0.2010.92 5.56 0.025 0.0029 0.0036 0.033 — 0.042 — — — — — — — — — — — — 602Conforming steel P 0.203 0.91 5.43 0.022 0.0026 0.0039 0.027 — — 0.0021— — — — — — — — — — — 606 Conforming steel Q 0.236 1.31 5.43 0.0260.0025 0.0034 0.035 — — — 0.345 — — — — — — — — — — 604 Conforming steelR 0.155 0.41 4.89 0.031 0.0023 0.0034 0.033 — — — — 0.245 — — — — — — —— — 619 Conforming steel S 0.161 0.77 4.73 0.029 0.0023 0.0034 0.030 — —— — — 0.044 — — — — — — — — 624 Conforming steel T 0.152 0.67 5.09 0.0310.0025 0.0038 0.039 — — — — — — 0.422 — — — — — — — 615 Conforming steelU 0.121 1.45 4.90 0.023 0.0029 0.0037 0.035 — — — — — — — 0.265 — — — —— — 626 Conforming steel V 0.189 0.89 4.89 0.030 0.0032 0.0036 0.036 — —— — — — — — 0.007 — — — — — 621 Conforming steel W 0.135 0.65 4.99 0.0260.0024 0.0037 0.044 — — — — — — — — — — 0.006 — — — 616 Conforming steelX 0.221 0.65 5.60 0.027 0.0029 0.0046 0.042 — 0.045 — — — — — — — — — —— — 598 Conforming steel Y 0.209 0.51 5.07 0.029 0.0030 0.0039 0.042 —0.049 — — — — — — 0.008 — — — — — 611 Conforming steel Z 0.209 0.35 5.630.027 0.0027 0.0046 0.031 — 0.041 — — — — — — — — 0.009 — — — 594Conforming steel AA 0.227 1.02 5.24 0.029 0.0028 0.0047 0.044 — — — — —— — — — — — 0.0021 — — 612 Conforming steel AB 0.218 1.42 5.22 0.0300.0020 0.0043 0.041 — — — — — — — — — — — — 0.0028 — 617 Conformingsteel AC 0.220 1.18 5.54 0.039 0.0026 0.0035 0.024 — — — — — — — — — — —— — 0.0020 605 Conforming steel AD 0.197 1.18 5.14 0.027 0.0028 0.00330.031 — — — — — — — — — 0.010 — — — — 617 Conforming steel Underline:outside range according to present disclosure

TABLE 2 (First) hot-rolled sheet heat treatment Steel Slab heatingFinisher delivery Average coiling Heat treatment Heat treatment sampletemperature temperature temperature temperature time No. ID (° C.) (°C.) (° C.) (° C.) (s)  1 A 1220 890 550 653 20000  2 B 1230 880 510 63915000  3 C 1190 880 610 642 14000  4 A 1220 705 550 642 18000  5 A 12301085  500 642 11000  6 A 1260 860 520 510 14000  7 A 1230 870 500 83018000  8 A 1260 880 580 642  100  9 A 1210 870 620 650 18000 10 A 1200870 620 647 19000 11 A 1100 890 550 650 18000 12 A 1300 880 480 66018000 13 A 1200 750 500 650 18000 14 A 1250 1000  620 700 11000 15 A1220 900 300 650 11000 16 A 1200 880 750 680 17000 17 A 1210 870 600 73317000 18 A 1220 900 500 650  600 19 A 1200 880 550 660 10000 20 A 1230890 450 680 10000 21 A 1210 870 500 680 12000 22 A 1190 850 480 70012000 23 A 1195 880 450 680 14000 24 A 1200 880 600 640 17000 25 A 1210870 620 650  5000 26 A 1220 870 620 652  6000 27 A 1230 870 600 642 6000 28 A 1200 870 570 642 11000 29 A 1220 850 570 642  9000 30 A 1190880 580 642 18000 31 D 1230 850 540 659 18000 32 E 1230 880 530 654 8000 33 F 1240 890 560 654 16000 34 G 1230 880 600 650  7000 35 H 1250850 570 666  8000 36 I 1230 910 610 642 15000 37 IA 1250 870 550 64517000 38 IB 1220 870 540 650 18000 39 IC 1250 870 450 650 18000 40 ID1230 850 550 665 17000 41 IE 1240 860 560 660 15000 42 IF 1200 850 530650 15000 43 IG 1210 860 540 650 14000 44 IH 1200 870 550 620 17000 45II 1220 860 500 640 18000 46 IJ 1250 860 510 640 14000 47 IK 1240 870510 650 18000 48 IL 1200 860 520 650 19000 49 IM 1210 850 460 650 1700050 IN 1195 860 480 630 18000 51 IO 1190 870 450 660 17000 52 IP 1200 850500 650 17000 53 J 1210 850 640 639 17000 54 K 1200 860 630 687 19000 55L 1230 830 580 735  2000 56 M 1240 820 550 638  5000 57 N 1250 840 590634  7000 58 O 1260 860 550 645 14000 59 P 1210 890 530 645 20000 60 Q1250 830 610 645 18000 61 R 1260 820 570 662 14000 62 S 1230 870 630 666 8000 63 T 1240 810 610 651  7000 64 U 1240 840 540 667 15000 65 V 1230910 580 659 13000 66 W 1220 900 510 656 10000 67 X 1240 880 600 64515000 68 Y 1250 890 530 652  9000 69 Z 1240 870 550 636 20000 70 AA 1250890 530 652  8000 71 AB 1240 870 550 658 15000 72 AC 1250 850 540 652 5000 73 AD 1230 840 530 659  8000 74 A 1220 890 550 651  1000 75 A 1230870 540 655  8000 (Second) annealing treatment Rolling reduction Heattreatment Heat treatment in cold rolling temperature time No. (%) (° C.)(s) Type* Remarks  1 25.9 643 350 GA Example  2 22.2 629 300 CR Example 3 25.0 632 250 CR Example  4 24.1 632 300 GI Comparative Example  521.4 632 350 EG Comparative Example  6 25.9 632 500 EG ComparativeExample  7 25.0 632 300 EG Comparative Example  8 18.5 632 500 CRComparative Example  9 19.2 630 550 CR Example 10  0.0 632 550 CRExample 11 13.8 640 300 CR Example 12 27.6 640 350 CR Example 13 22.2632 350 GA Example 14 14.3 640 400 GA Example 15 13.8 640 300 GA Example16  7.4 632 350 GA Example 17 22.2 632 350 GA Example 18 17.9 632 500 GAExample 19 29.1 640 350 GA Example 20 27.6 713 150 GA Example 21 20.7660  20 GA Example 22  7.4 630 900 GA Example 23 31.0 640 350 CRComparative Example 24 12.5 640 1000  GA Comparative Example 25 70.2 635550 CR Comparative Example 26 42.9 635 550 CR Comparative Example 2723.1 520 750 CR Comparative Example 28 28.6 800 400 GI ComparativeExample 29 25.0 632  3 CR Comparative Example 30 22.2 632 2200  CRComparative Example 31 25.9 649 500 CR Example 32 28.6 644 600 GIExample 33 25.0 644 600 CR Example 34 25.9 640 550 CR Example 35 21.4656 650 CR Example 36 21.4 632 300 GA Example 37 22.2 635 350 CR Example38 19.2 635 300 CR Example 39 19.2 635 350 GA Example 40 18.5 655 350 CRExample 41 25.9 650 500 GA Example 42 25.0 640 450 CR Example 43 16.0640 500 CR Example 44 15.4 640 500 GA Example 45 25.0 640 350 GA Example46 22.2 635 350 CR Example 47 21.4 635 400 CR Example 48 25.0 635 400 CRExample 49 25.9 640 400 GA Example 50 25.0 640 400 GA Example 51 17.9655 350 CR Comparative Example 52 18.5 635 500 CR Comparative Example 5325.9 629 250 CR Comparative Example 54 25.0 677 200 EG ComparativeExample 55 25.9 725 300 CR Comparative Example 56 26.9 628 400 EGComparative Example 57 25.9 624 150 GI Example 58 25.0 635 100 CRExample 59 22.2 635 200 GA Example 60 25.9 635 320 CR Example 61 26.9652 300 CR Example 62 28.0 656 300 Al Example 63 25.9 641 200 GI Example64 25.9 657 250 GI Example 65 25.0 649 350 GI Example 66 24.1 646 300 EGExample 67 22.2 635 300 Al Example 68 25.9 642 340 GA Example 69 26.9626 500 CR Example 70 26.9 642 500 Al Example 71 28.6 648 400 GI Example72 25.9 642 350 CR Example 73 25.0 648 300 CR Example 74 25.9 625 200 CRExample 75 22.2 624 250 CR Example Underline: outside range according topresent disclosure *CR: cold-rolled steel sheet (no coating), GI:hot-dip galvanized steel sheet (no galvannealing), GA: galvannealedsteel sheet Al: hot-dip aluminum-coated steel sheet, EG:electrogalvanized steel sheet

TABLE 3 Average Aspect Rolling Area Area Volume grain ratio of Steelreduction in ratio Area ratio ratio fraction Area ratio size crystalsample cold rolling of F of F′ of M of RA of ε (μm) grains No. ID (%)(%) (%) (%) (%) (%) F M RA F M RA Balance Remarks 1 A 25.9 30.5 20.119.1 24.5 2.2 1.8 0.9 0.8 6.9 5.5 5.9 P, θ, ε Example 2 B 22.2 25.3 26.116.1 29.8 1.8 1.7 0.7 0.6 7.3 5.1 6.3 P, θ, ε Example 3 C 25.0 13.6 30.116.8 36.8 2.1 1.6 0.8 1.1 7.8 5.3 5.7 P, θ, ε Example 4 A 24.1 33.1 18.633.2  9.2 0.9 2.1 2.5 1.3 6.5 5.1 6.5 P, θ, ε Comparative Example 5 A21.4 30.5 18.4 35.1  8.9 1.2 2.2 2.2 1.2 6.1 5.0 5.4 P, θ, ε ComparativeExample 6 A 25.9 32.1 19.2 19.1 23.4 2.1 2.1 1.3 1.2 7.1 5.3 5.4 P, θ, εComparative Example 7 A 25.0 33.8 19.2 18.9 24.5 1.8 2.2 1.2 1.1 6.5 5.46.1 P, θ, ε Comparative Example 8 A 18.5 30.8 19.1 18.0 24.9 1.6 2.1 1.11.0 5.5 5.3 6.3 P, θ, ε Comparative Example 9 A 19.2 30.4 19.4 22.5 25.01.3 1.5 1.1 0.8 8.1 5.9 10.2  P, θ, ε Example 10 A  0.0 28.1 19.4 23.424.8 1.2 2.1 1.0 0.9 15.0  14.8  16.1  P, θ, ε Example 11 A 13.8 25.022.0 25.0 23.0 1.0 1.5 1.1 0.9 8.2 6.1 6.1 P, θ, ε Example 12 A 27.631.0 18.0 24.0 24.5 1.2 2.5 1.8 1.5 6.9 5.1 5.2 P, θ, ε Example 13 A22.2 30.5 19.4 22.4 24.8 1.3 2.1 1.1 0.9 7.2 5.5 5.5 P, θ, ε Example 14A 14.3 30.1 19.2 22.0 24.1 1.5 2.1 1.1 0.8 8.4 5.4 6.2 P, θ, ε Example15 A 13.8 29.1 18.9 22.5 23.8 1.6 2.2 1.0 0.9 8.4 5.9 6.1 P, θ, εExample 16 A  7.4 29.4 19.1 23.5 24.1 1.5 2.1 1.1 0.9 12.1  10.5  11.5 P, θ, ε Example 17 A 22.2 10.5 27.4 28.2 23.0 1.5 1.8 1.5 1.2 7.5 5.85.2 P, θ, ε Example 18 A 17.9 25.1 15.8 28.9 21.0 2.0 1.9 1.2 0.9 7.55.9 6.1 P, θ, ε Example 19 A 29.1 32.1 19.0 19.4 23.0 1.5 2.5 1.1 1.24.1 4.5 5.1 P, θ, ε Example 20 A 27.6 32.5 15.4 18.9 21.4 2.0 3.1 1.20.9 5.3 6.1 5.7 P, θ, ε Example 21 A 20.7 18.9 25.4 23.0 22.7 1.8 1.91.5 1.2 6.9 5.8 5.1 P, θ, ε Example 22 A  7.4 15.2 36.9 23.8 21.8 1.13.5 1.2 0.9 15.4  11.1  11.2  P, θ, ε Example 23 A 31.0 38.7 15.4 22.120.4 1.8 3.8 1.9 1.2 22.0  19.8  21.1  P, θ, ε Comparative Example 24 A12.5 52.0  5.0 15.4 15.8 1.2 5.1 2.1 0.8 6.9 5.4 5.1 P, θ, ε ComparativeExample 25 A 70.2 50.7 12.0 20.8  9.1 1.5 4.2 2.0 0.5 1.4 1.4 1.5 P, θ,ε Comparative Example 26 A 42.9 37.8 19.4 21.7 12.1 2.0 3.8 1.8 0.5 1.91.8 1.9 P, θ, ε Comparative Example 27 A 23.1 34.2 15.3 33.1 10.8 1.64.5 2.3 2.1 7.2 5.2 5.9 P, θ, ε Comparative Example 28 A 28.6 35.2 18.232.4 10.4 1.3 4.6 2.1 2.4 7.1 5.5 6.6 P, θ, ε Comparative Example 29 A25.0 33.8 19.1 27.8  9.8 1.7 4.2 2.3 2.2 7.9 5.0 5.7 P, θ, ε ComparativeExample 30 A 22.2 32.9  5.1 18.2 24.5 1.8 1.7 3.9 3.8 5.1 4.8 5.1 P, θ,ε Comparative Example 31 D 25.9 24.8 24.1 20.1 28.9 2.1 1.6 0.8 0.6 6.94.2 6.0 P, θ, ε Example 32 E 28.6 33.8 19.7 19.4 25.4 0.6 1.9 1.0 0.98.1 5.1 6.8 P, θ, ε Example 33 F 25.0 30.2 22.5 17.7 26.8 2.1 1.5 0.80.7 7.1 5.1 6.4 P, θ, ε Example 34 G 25.9 13.7 28.9 15.4 32.9 4.2 1.50.7 0.8 7.4 5.2 5.9 P, θ, ε Example 35 H 21.4 30.4 20.3 18.9 26.8 2.11.7 0.7 0.6 7.6 5.9 6.1 P, θ, ε Example 36 I 21.4 32.6 20.5 17.5 27.80.8 1.6 0.6 0.5 7.7 5.8 6.2 P, θ, ε Example 37 IA 22.2 35.1 12.0 19.225.1 0.8 2.2 1.1 1.2 6.8 6.1 5.7 P, θ, ε Example 38 IB 19.2 10.5 15.028.5 25.4 0.9 2.3 1.1 0.9 6.9 5.5 5.9 P, θ, ε Example 39 IC 19.2 20.518.9 19.1 23.8 1.5 2.2 1.2 1.1 7.2 6.1 6.2 P, θ, ε Example 40 ID 18.532.5 20.5 19.9 24.0 1.5 2.1 1.0 1.2 7.5 5.8 6.4 P, θ, ε Example 41 IE25.9 33.6 20.3 18.9 23.1 1.5 1.9 1.5 1.2 8.1 5.8 5.9 P, θ, ε Example 42IF 25.0 12.8 20.6 22.5 25.4 1.2 1.9 1.4 1.1 6.9 5.9 6.4 P, θ, ε Example43 IG 16.0 30.5 22.1 19.2 24.1 1.5 2.0 1.2 0.8 6.8 6.1 6.1 P, θ, εExample 44 IH 15.4 32.1 21.5 18.9 25.1 1.2 2.1 1.2 0.9 7.2 5.5 6.7 P, θ,ε Example 45 II 25.0 31.5 20.5 18.9 25.1 1.2 2.2 1.3 0.9 7.4 5.7 5.8 P,θ, ε Example 46 IJ 22.2 30.4 20.6 19.1 24.1 1.2 2.0 1.1 1.1 7.1 5.9 5.9P, θ, ε Example 47 IK 21.4 32.0 25.1 15.2 22.5 1.5 2.1 1.5 1.0 6.9 6.16.4 P, θ, ε Example 48 IL 25.0 20.1 25.1 28.5 23.7 1.1 1.8 1.4 1.1 6.85.9 6.2 P, θ, ε Example 49 IM 25.9 35.6 18.1 23.1 22.1 0.8 1.8 1.2 0.96.7 6.1 5.9 P, θ, ε Example 50 IN 25.0 15.4 18.9 25.8 25.4 1.8 1.9 1.11.1 7.1 5.9 5.9 P, θ, ε Example 51 IO 17.9 35.8 18.2 18.9 22.0 0.9 2.22.0 1.1 21.5  22.0  23.1  P, θ, ε Comparative Example 52 IP 18.5 38.122.8 20.5  5.5 1.8 1.1 1.5 1.4 3.0 4.5 5.2 P, θ, ε Comparative Example53 J 25.9 72.8  8.6  8.5  8.4 0.7 6.7 0.4 0.3 2.1 3.8 3.5 P, θ, εComparative Example 54 K 25.0 50.0 15.6 19.1 12.4 1.8 4.3 1.7 1.6 3.12.9 3.8 P, θ, ε Comparative Example 55 L 25.9 62.5 14.1 15.1  8.1 0.25.8 3.7 3.6 2.3 3.1 3.1 P, θ, ε Comparative Example 56 M 26.9 58.1  3.919.4 13.2 1.9 6.2 3.5 3.4 2.0 3.1 2.5 P, θ, ε Comparative Example 57 N25.9 30.1 20.4 19.2 26.5 0.6 1.7 1.1 0.7 7.3 5.9 6.4 P, θ, ε Example 58O 25.0 29.7 22.3 18.1 28.1 0.9 1.5 0.8 0.7 7.2 6.1 5.9 P, θ, ε Example59 P 22.2 30.5 20.5 18.2 24.5 2.5 1.3 0.6 0.7 6.9 5.7 6.4 P, θ, εExample 60 Q 25.9 30.1 19.8 18.1 26.4 3.1 1.2 0.8 0.6 8.0 6.2 5.1 P, θ,ε Example 61 R 26.9 32.1 20.4 18.6 27.5 0.1 1.2 0.5 0.7 5.9 5.1 6.5 P,θ, ε Example 62 S 28.0 31.2 22.4 18.2 26.8 0.7 1.4 0.8 0.8 7.3 5.7 4.9P, θ, ε Example 63 T 25.9 30.5 20.1 18.1 27.5 0.0 0.9 0.7 0.9 8.2 5.55.4 P, θ, ε Example 64 U 25.9 31.8 21.0 18.2 26.8 2.1 1.4 0.8 0.7 5.95.2 6.5 P, θ, ε Example 65 V 25.0 31.8 19.7 17.8 25.9 0.6 1.2 0.8 0.77.1 6.1 5.4 P, θ, ε Example 66 W 24.1 22.8 27.1 17.7 28.5 0.7 1.4 0.60.7 6.5 4.1 6.1 P, θ, ε Example 67 X 22.2 25.3 28.1 17.3 26.4 2.1 1.30.6 0.7 6.8 5.2 5.7 P, θ, ε Example 68 Y 25.9 20.4 26.3 17.1 32.1 2.21.5 0.6 0.5 6.9 6.1 6.4 P, θ, ε Example 69 Z 26.9 30.4 19.7 18.4 29.40.9 1.4 0.9 0.7 7.2 5.8 6.3 P, θ, ε Example 70 AA 26.9 30.8 20.1 16.226.8 2.6 1.3 0.7 0.8 7.1 6.1 7.1 P, θ, ε Example 71 AB 28.6 31.0 21.518.1 27.4 0.6 1.4 0.6 0.5 6.9 6.5 5.9 P, θ, ε Example 72 AC 25.9 30.219.1 17.8 28.4 2.2 1.2 0.8 0.5 7.4 5.9 5.8 P, θ, ε Example 73 AD 25.025.7 20.1 18.1 29.8 0.7 1.1 0.6 0.4 8.4 6.1 6.4 P, θ, ε Example 74 A25.9 32.1 21.4 19.4 22.4 3.1 2.2 1.0 0.8 8.1 4.6 7.2 P, θ, ε Example 75A 22.2 31.8 18.9 19.5 21.8 2.4 2.3 0.9 0.7 7.9 4.4 5.8 P, θ, ε ExampleUnderline: outside range according to present disclosure F: polygonalferrite, F′: non-recrystallized ferrite, RA: retained austenite, M:martensite, P: pearlite, θ: carbide (such as cementite) ε: ε phase withhep structure

TABLE 4 0.04 × 0.04 × Average Value obtained by dividing Average MnAverage Mn (Mn content in RA) + (Mn content in RA) + C content RAremaining volume fraction Steel sample content in RA content in FAverage Mn content in RA/ 0.056 − 0.180 0.056 + 0.180 in RA after 10%tensile working by No. ID (mass %) (mass %) Average Mn content in F(mass %) (mass %) (mass %) RA volume fraction before working Remarks 1 A8.78 3.18 2.76 0.23 0.59 0.43 0.72 Example 2 B 9.12 3.11 2.93 0.24 0.600.40 0.78 Example 3 C 9.03 3.15 2.87 0.24 0.60 0.45 0.67 Example 4 A8.11 3.15 2.57 0.20 0.56 0.28 0.44 Comparative Example 5 A 8.02 3.252.47 0.20 0.56 0.18 0.49 Comparative Example 6 A 6.41 3.36 1.91 0.130.49 0.12 0.38 Comparative Example 7 A 6.84 3.49 1.96 0.15 0.51 0.140.46 Comparative Example 8 A 6.94 3.59 1.93 0.15 0.51 0.23 0.42Comparative Example 9 A 7.54 3.18 2.37 0.18 0.54 0.25 0.52 Example 10 A8.10 3.25 2.49 0.20 0.56 0.32 0.75 Example 11 A 8.20 2.97 2.76 0.20 0.560.33 0.65 Example 12 A 8.50 2.99 2.84 0.22 0.58 0.35 0.70 Example 13 A8.50 3.02 2.81 0.22 0.58 0.39 0.78 Example 14 A 8.90 3.11 2.86 0.23 0.590.28 0.74 Example 15 A 8.50 3.05 2.79 0.22 0.58 0.31 0.65 Example 16 A8.90 3.12 2.85 0.23 0.59 0.29 0.68 Example 17 A 8.70 3.12 2.79 0.22 0.580.41 0.71 Example 18 A 8.44 3.01 2.80 0.21 0.57 0.33 0.66 Example 19 A9.10 3.15 2.89 0.24 0.60 0.45 0.69 Example 20 A 8.50 3.08 2.76 0.22 0.580.38 0.74 Example 21 A 7.51 3.14 2.39 0.18 0.54 0.39 0.78 Example 22 A9.45 3.11 3.04 0.25 0.61 0.41 0.81 Example 23 A 7.45 3.02 2.47 0.17 0.530.55 0.29 Comparative Example 24 A 8.99 3.08 2.92 0.24 0.60 0.61 0.29Comparative Example 25 A 8.45 3.87 2.18 0.21 0.57 0.19 0.21 ComparativeExample 26 A 8.56 3.22 2.66 0.22 0.58 0.19 0.27 Comparative Example 27 A6.88 4.02 1.71 0.15 0.51 0.14 0.38 Comparative Example 28 A 6.48 4.061.60 0.14 0.50 0.13 0.47 Comparative Example 29 A 6.29 3.98 1.58 0.130.49 0.21 0.46 Comparative Example 30 A 6.01 3.18 1.89 0.12 0.48 0.200.52 Comparative Example 31 D 9.02 3.05 2.96 0.24 0.60 0.42 0.74 Example32 E 8.56 3.22 2.66 0.22 0.58 0.40 0.82 Example 33 F 8.35 3.12 2.68 0.210.57 0.42 0.75 Example 34 G 9.12 3.09 2.95 0.24 0.60 0.39 0.67 Example35 H 9.06 3.05 2.97 0.24 0.60 0.38 0.69 Example 36 I 9.15 3.09 2.96 0.240.60 0.44 0.70 Example 37 IA 7.45 3.21 2.32 0.17 0.53 0.33 0.35 Example38 IB 5.88 2.85 2.06 0.11 0.47 0.46 0.78 Example 39 IC 6.48 3.23 2.010.14 0.50 0.42 0.65 Example 40 ID 7.88 3.91 2.02 0.19 0.55 0.45 0.81Example 41 IE 7.68 2.51 3.06 0.18 0.54 0.44 0.71 Example 42 IF 8.59 4.212.04 0.22 0.58 0.41 0.68 Example 43 IG 8.46 3.56 2.38 0.21 0.57 0.510.66 Example 44 IH 9.12 3.21 2.84 0.24 0.60 0.50 0.80 Example 45 II 9.013.14 2.87 0.24 0.60 0.51 0.74 Example 46 IJ 9.45 3.02 3.13 0.25 0.610.48 0.71 Example 47 IK 9.15 3.18 2.88 0.24 0.60 0.42 0.68 Example 48 IL8.59 3.55 2.42 0.22 0.58 0.44 0.72 Example 49 IM 9.01 4.02 2.24 0.240.60 0.45 0.51 Example 50 IN 8.78 3.58 2.45 0.23 0.59 0.51 0.55 Example51 IO 6.54 2.55 2.56 0.14 0.50 0.33 0.18 Comparative Example 52 IP 6.883.98 1.73 0.15 0.51 0.35 0.25 Comparative Example 53 J 8.01 3.11 2.580.20 0.56 0.38 0.49 Comparative Example 54 K 8.11 3.22 2.52 0.20 0.560.28 0.47 Comparative Example 55 L 3.52 2.08 1.69 0.02 0.38 0.10 0.52Comparative Example 56 M 8.04 3.34 2.41 0.20 0.56 0.12 0.44 ComparativeExample 57 N 8.78 3.22 2.73 0.23 0.59 0.42 0.81 Example 58 O 8.24 3.142.62 0.21 0.57 0.42 0.74 Example 59 P 8.68 3.29 2.64 0.22 0.58 0.37 0.68Example 60 Q 8.78 3.09 2.84 0.23 0.59 0.43 0.74 Example 61 R 8.88 3.052.91 0.23 0.59 0.39 0.73 Example 62 S 8.15 3.27 2.49 0.20 0.56 0.38 0.83Example 63 T 8.25 3.24 2.55 0.21 0.57 0.39 0.64 Example 64 U 8.33 3.192.61 0.21 0.57 0.39 0.69 Example 65 V 8.35 3.18 2.63 0.21 0.57 0.42 0.84Example 66 W 9.22 3.09 2.98 0.24 0.60 0.39 0.76 Example 67 X 9.04 3.282.76 0.24 0.60 0.38 0.72 Example 68 Y 9.12 3.04 3.00 0.24 0.60 0.44 0.78Example 69 Z 8.49 3.11 2.73 0.22 0.58 0.38 0.74 Example 70 AA 8.57 3.082.78 0.22 0.58 0.43 0.77 Example 71 AB 8.67 3.21 2.70 0.22 0.58 0.370.78 Example 72 AC 8.18 3.18 2.57 0.20 0.56 0.42 0.72 Example 73 AD 8.923.09 2.89 0.23 0.59 0.43 0.69 Example 74 A 8.27 3.34 2.48 0.21 0.57 0.200.29 Example 75 A 8.19 3.56 2.30 0.20 0.56 0.19 0.28 Example Underline:outside range according to present disclosure F: polygonal ferrite, RA:retained austenite, M: martensite

TABLE 5 Sheet Steel sample thickness Sheet passage ability Sheet passageability Surface characteristics of YP YR TS EL TS × EL λ No. ID (mm)during hot rolling during cold rolling final-annealed sheet Productivity(MPa) (%) (MPa) (%) (MPa · %) R/t (%) Remarks 1 A 2.0 Good Good GoodGood 985 96.8 1018 32.2 32780 0.3 42 Example 2 B 2.1 Good Good Good Good1102 91.5 1204 28.4 34194 0.5 35 Example 3 C 2.1 Good Good Good Good1205 80.9 1489 20.2 30078 1.0 25 Example 4 A 2.2 Poor Poor Poor Poor 83482.5 1011 19.8 20018 0.9 15 Comparative Example 5 A 2.2 Good Poor PoorPoor 846 83.8 1009 20.4 20584 0.9 18 Comparative Example 6 A 2.0 GoodPoor Good Good 812 81.0 1002 20.8 20842 0.6 21 Comparative Example 7 A2.1 Good Good Good Good 814 79.0 1031 19.7 20311 0.6 23 ComparativeExample 8 A 2.2 Good Poor Good Good 798 79.4 1005 18.9 18995 0.6 21Comparative Example 9 A 2.1 Good Good Good Good 985 96.8 1018 28.2 287081.0 29 Example 10 A 2.8 Good Good Good Good 974 95.7 1018 29.1 29624 0.735 Example 11 A 2.5 Good Good Good Good 856 81.4 1051 27.1 28482 0.8 35Example 12 A 2.1 Good Good Good Good 867 84.7 1024 26.0 26624 1.0 32Example 13 A 2.1 Good Good Good Good 920 91.1 1010 26.8 27068 1.0 35Example 14 A 2.4 Good Good Good Good 912 89.6 1018 26.7 27181 0.8 38Example 15 A 2.5 Good Good Good Good 950 87.9 1081 28.9 31241 0.8 41Example 16 A 2.5 Good Good Good Good 1020 99.5 1025 27.1 27778 0.8 35Example 17 A 2.1 Good Good Good Good 980 88.9 1102 26.1 28762 1.0 49Example 18 A 2.3 Good Good Good Good 990 94.1 1052 28.0 29456 0.9 35Example 19 A 2.1 Good Good Good Good 780 74.2 1051 31.5 33107 1.0 38Example 20 A 2.1 Good Good Good Good 851 83.4 1020 26.1 26622 1.0 37Example 21 A 2.3 Good Good Good Good 840 74.0 1135 26.1 29624 0.9 32Example 22 A 2.5 Good Good Good Good 852 80.6 1057 26.1 27588 0.8 34Example 23 A 2.0 Good Good Good Good 751 79.1 950 31.0 29450 1.0 36Comparative Example 24 A 2.1 Good Good Good Good 1002 83.5 1200 19.823760 1.0 31 Comparative Example 25 A 1.4 Good Good Good Good 822 81.41010 35.1 35451 3.2 15 Comparative Example 26 A 2.0 Good Good Good Good854 83.9 1018 18.9 19240 2.3 20 Comparative Example 27 A 2.0 Good GoodGood Good 789 77.4 1019 18.4 18750 0.5 28 Comparative Example 28 A 2.0Good Good Good Poor 814 80.1 1016 17.9 18186 0.5 30 Comparative Example29 A 2.1 Good Good Good Good 813 77.9 1044 18.6 19418 0.5 25 ComparativeExample 30 A 2.1 Good Good Poor Poor 600 58.8 1020 29.8 30396 0.9 10Comparative Example 31 D 2.0 Good Good Good Good 1054 86.1 1224 26.932926 0.5 30 Example 32 E 2.0 Good Good Good Good 815 81.7 998 31.831736 0.4 35 Example 33 F 2.1 Good Good Good Good 812 79.8 1018 29.830336 0.4 42 Example 34 G 2.0 Good Good Good Good 1251 84.4 1482 21.531863 1.0 22 Example 35 H 2.2 Good Good Good Good 821 81.2 1011 32.532858 0.5 35 Example 36 I 2.2 Good Good Good Good 819 82.3 995 31.931741 0.5 38 Example 37 IA 2.1 Good Good Good Good 850 86.6 981 27.126585 1.0 35 Example 38 IB 2.1 Good Good Good Good 900 74.8 1203 22.126586 1.0 32 Example 39 IC 2.1 Good Good Good Good 920 90.4 1018 26.827282 1.0 35 Example 40 ID 2.2 Good Good Good Good 910 89.2 1020 26.727234 0.9 38 Example 41 IE 2.0 Good Good Good Good 750 75.8 990 35.435046 1.0 41 Example 42 IF 2.1 Good Good Good Good 1100 74.6 1474 18.427122 1.0 35 Example 43 IG 2.1 Good Good Good Good 980 89.1 1100 28.130910 1.0 49 Example 44 IH 2.2 Good Good Good Good 990 82.2 1204 22.426970 0.9 35 Example 45 II 2.1 Good Good Good Good 780 78.2 998 31.531437 1.0 38 Example 46 IJ 2.1 Good Good Good Good 851 83.4 1020 26.126622 1.0 37 Example 47 IK 2.2 Good Good Good Good 840 70.9 1185 25.430099 0.9 32 Example 48 IL 2.1 Good Good Good Good 852 80.6 1057 26.127588 1.0 34 Example 49 IM 2.0 Good Good Good Good 754 68.1 1107 27.530443 1.0 33 Example 50 IN 2.1 Good Good Good Good 1002 98.2 1020 26.827336 1.0 35 Example 51 IO 2.3 Good Good Good Good 751 84.4 890 31.027590 0.9 36 Comparative Example 52 IP 2.2 Good Good Good Good 1002 83.51200 17.5 21000 0.9 31 Comparative Example 53 J 2.0 Good Good Good Good721 87.6 823 22.4 18435 0.4 42 Comparative Example 54 K 2.1 Good GoodPoor Good 1245 71.0 1754 10.4 18242 1.4 9 Comparative Example 55 L 2.0Good Good Good Good 624 61.3 1018 18.2 18528 1.5 15 Comparative Example56 M 1.9 Good Good Good Good 602 60.3 998 24.9 24850 1.3 16 ComparativeExample 57 N 2.0 Good Good Good Good 819 80.5 1018 32.4 32983 0.4 35Example 58 O 2.1 Good Good Good Good 798 77.9 1024 31.5 32256 0.4 37Example 59 P 2.1 Good Good Good Good 834 80.7 1034 30.5 31537 0.4 32Example 60 Q 2.0 Good Good Good Good 845 85.4 989 31.8 31450 0.4 38Example 61 R 1.9 Good Good Good Good 860 82.6 1041 31.9 33208 0.4 31Example 62 S 1.8 Good Good Good Good 796 77.2 1031 32.9 33920 0.4 34Example 63 T 2.0 Good Good Good Good 814 82.3 989 31.4 31055 0.4 39Example 64 U 2.0 Good Good Good Good 816 79.0 1033 32.8 33882 0.4 34Example 65 V 2.1 Good Good Good Good 809 79.0 1024 33.6 34406 0.4 38Example 66 W 2.2 Good Good Good Good 1009 83.3 1212 26.8 32482 0.5 25Example 67 X 2.1 Good Good Good Good 957 79.9 1198 27.1 32466 0.5 29Example 68 Y 2.0 Good Good Good Good 968 79.0 1225 25.9 31728 0.5 29Example 69 Z 1.9 Good Good Good Good 804 77.1 1043 33.5 34941 0.4 45Example 70 AA 1.9 Good Good Good Good 825 81.0 1019 32.6 33219 0.4 40Example 71 AB 2.0 Good Good Good Good 789 75.4 1046 31.5 32949 0.4 41Example 72 AC 2.0 Good Good Good Good 814 81.5 999 30.8 30769 0.4 41Example 73 AD 2.1 Good Good Good Good 829 75.9 1092 34.2 37346 0.8 38Example 74 A 2.0 Good Good Good Good 834 84.3 989 27.2 26901 0.8 42Example 75 A 2.1 Good Good Good Good 819 82.3 995 26.8 26666 0.7 30Example

From above, it can be seen that the steel sheets according to thedisclosure each exhibited TS of 980 MPa or more and YR of 68% or more,and are thus considered as high-strength steel sheets having excellentformability and high yield ratio and hole expansion formability. Incontrast, the comparative examples are inferior in terms of at least oneof YR, TS, EL, λ, and R/t. Moreover, each steel sheet containing ε phaseat an area ratio of 2% or more achieved a still better balance betweenstrength and ductility.

INDUSTRIAL APPLICABILITY

According to the disclosure, it becomes possible to manufacturehigh-strength steel sheets with excellent formability and high yieldratio and hole expansion formability that exhibit TS of 980 MPa or moreand YR of 68% or more and that satisfy the condition of TS*EL≥22,000 MPa%. Steel sheets according to the disclosure are highly beneficial inindustrial terms, because they can improve fuel efficiency when appliedto, for example, automobile structural parts, by a reduction in theweight of automotive bodies.

1-7. (canceled)
 8. A steel sheet comprising: a chemical compositioncontaining, in mass %, C: 0.030% or more and 0.250% or less, Si: 0.01%or more and 3.00% or less, Mn: more than 4.20% and 6.00% or less, P:0.001% or more and 0.100% or less, S: 0.0200% or less, N: 0.0100% orless, and Ti: 0.005% or more and 0.200% or less, and optionally furthercontaining, in mass %, at least one selected from the group consistingof Al: 0.01% or more and 2.00% or less, Nb: 0.005% or more and 0.200% orless, B: 0.0003% or more and 0.0050% or less, Ni: 0.005% or more and1.000% or less, Cr: 0.005% or more and 1.000% or less, V: 0.005% or moreand 0.500% or less, Mo: 0.005% or more and 1.000% or less, Cu: 0.005% ormore and 1.000% or less, Sn: 0.002% or more and 0.200% or less, Sb:0.002% or more and 0.200% or less, Ta: 0.001% or more and 0.010% orless, Ca: 0.0005% or more and 0.0050% or less, Mg: 0.0005% or more and0.0050% or less, and REM: 0.0005% or more and 0.0050% or less, with thebalance consisting of Fe and inevitable impurities; and a steelmicrostructure that contains, in area ratio, 5% or more and 50% or lessof polygonal ferrite, 10% or more of non-recrystallized ferrite, and 15%or more and 30% or less of martensite, and that contains, in volumefraction, 12% or more of retained austenite, where an average aspectratio of crystal grains of each of the polygonal ferrite, themartensite, and the retained austenite is 2.0 or more and 20.0 or less,wherein the polygonal ferrite has an average grain size of 4 μm or less,the martensite has an average grain size of 2 μm or less, the retainedaustenite has an average grain size of 2 μm or less, and a valueobtained by dividing a Mn content in the retained austenite in mass % bya Mn content in the polygonal ferrite in mass % equals 2.0 or more. 9.The steel sheet according to claim 8, wherein the steel microstructurefurther contains, in area ratio, 2% or more of ε phase with an hcpstructure.
 10. The steel sheet according to claim 8, wherein theretained austenite has a C content that satisfies the following formulain relation to the Mn content in the retained austenite:0.04*[Mn]+0.056−0.180≤[C]≤0.04*[Mn]+0.056+0.180 where [C] is the Ccontent in the retained austenite in mass %, and [Mn] is the Mn contentin the retained austenite in mass %.
 11. The steel sheet according toclaim 9, wherein the retained austenite has a C content that satisfiesthe following formula in relation to the Mn content in the retainedaustenite:0.04*[Mn]+0.056−0.180≤[C]≤0.04*[Mn]+0.056+0.180 where [C] is the Ccontent in the retained austenite in mass %, and [Mn] is the Mn contentin the retained austenite in mass %.
 12. A coated steel sheetcomprising: the steel sheet according to claim 8; and one selected froma hot-dip galvanized layer, a galvannealed layer, a hot-dipaluminum-coated layer, and an electrogalvanized layer provided on thesteel sheet.
 13. A coated steel sheet comprising: the steel sheetaccording to claim 9; and one selected from a hot-dip galvanized layer,a galvannealed layer, a hot-dip aluminum-coated layer, and anelectrogalvanized layer provided on the steel sheet.
 14. A coated steelsheet comprising: the steel sheet according to claim 10; and oneselected from a hot-dip galvanized layer, a galvannealed layer, ahot-dip aluminum-coated layer, and an electrogalvanized layer providedon the steel sheet.
 15. A coated steel sheet comprising: the steel sheetaccording to claim 11; and one selected from a hot-dip galvanized layer,a galvannealed layer, a hot-dip aluminum-coated layer, and anelectrogalvanized layer provided on the steel sheet.
 16. A method formanufacturing the steel sheet according to claim 8, the methodcomprising: (i) heating a steel slab having a chemical compositioncontaining, in mass %, C: 0.030% or more and 0.250% or less, Si: 0.01%or more and 3.00% or less, Mn: more than 4.20% and 6.00% or less, P:0.001% or more and 0.100% or less, S: 0.0200% or less, N: 0.0100% orless, and Ti: 0.005% or more and 0.200% or less, and optionally furthercontaining, in mass %, at least one selected from the group consistingof Al: 0.01% or more and 2.00% or less, Nb: 0.005% or more and 0.200% orless, B: 0.0003% or more and 0.0050% or less, Ni: 0.005% or more and1.000% or less, Cr: 0.005% or more and 1.000% or less, V: 0.005% or moreand 0.500% or less, Mo: 0.005% or more and 1.000% or less, Cu: 0.005% ormore and 1.000% or less, Sn: 0.002% or more and 0.200% or less, Sb:0.002% or more and 0.200% or less, Ta: 0.001% or more and 0.010% orless, Ca: 0.0005% or more and 0.0050% or less, Mg: 0.0005% or more and0.0050% or less, and REM: 0.0005% or more and 0.0050% or less, with thebalance consisting of Fe and inevitable impurities; (ii) hot rolling thesteel slab with a finisher delivery temperature of 750° C. or higher and1000° C. or lower to obtain a steel sheet; (iii) coiling the steelsheet; (iv) then subjecting the steel sheet to pickling to removescales; (v) retaining the steel sheet in a temperature range of Ac₁[transformation temperature+20° C.] to [Ac₁ transformationtemperature+120° C.] for 600 s to 21,600 s; (vi) optionally cold rollingthe steel sheet at a rolling reduction of less than 30%; and (vii) thenretaining the steel sheet in a temperature range of [Ac₁ transformationtemperature+10° C.] to [Ac₁ transformation temperature+100° C.] for 20 sto 900 s and subsequently cooling the steel sheet.
 17. A method formanufacturing the steel sheet according to claim 9, the methodcomprising: (i) heating a steel slab having a chemical compositioncontaining, in mass %, C: 0.030% or more and 0.250% or less, Si: 0.01%or more and 3.00% or less, Mn: more than 4.20% and 6.00% or less, P:0.001% or more and 0.100% or less, S: 0.0200% or less, N: 0.0100% orless, and Ti: 0.005% or more and 0.200% or less, and optionally furthercontaining, in mass %, at least one selected from the group consistingof Al: 0.01% or more and 2.00% or less, Nb: 0.005% or more and 0.200% orless, B: 0.0003% or more and 0.0050% or less, Ni: 0.005% or more and1.000% or less, Cr: 0.005% or more and 1.000% or less, V: 0.005% or moreand 0.500% or less, Mo: 0.005% or more and 1.000% or less, Cu: 0.005% ormore and 1.000% or less, Sn: 0.002% or more and 0.200% or less, Sb:0.002% or more and 0.200% or less, Ta: 0.001% or more and 0.010% orless, Ca: 0.0005% or more and 0.0050% or less, Mg: 0.0005% or more and0.0050% or less, and REM: 0.0005% or more and 0.0050% or less, with thebalance consisting of Fe and inevitable impurities; (ii) hot rolling thesteel slab with a finisher delivery temperature of 750° C. or higher and1000° C. or lower to obtain a steel sheet; (iii) coiling the steelsheet; (iv) then subjecting the steel sheet to pickling to removescales; (v) retaining the steel sheet in a temperature range of Ac₁[transformation temperature+20° C.] to [Ac₁ transformationtemperature+120° C.] for 600 s to 21,600 s; (vi) optionally cold rollingthe steel sheet at a rolling reduction of less than 30%; and (vii) thenretaining the steel sheet in a temperature range of [Ac₁ transformationtemperature+10° C.] to [Ac₁ transformation temperature+100° C.] for 20 sto 900 s and subsequently cooling the steel sheet.
 18. A method formanufacturing the steel sheet according to claim 10, the methodcomprising: (i) heating a steel slab having a chemical compositioncontaining, in mass %, C: 0.030% or more and 0.250% or less, Si: 0.01%or more and 3.00% or less, Mn: more than 4.20% and 6.00% or less, P:0.001% or more and 0.100% or less, S: 0.0200% or less, N: 0.0100% orless, and Ti: 0.005% or more and 0.200% or less, and optionally furthercontaining, in mass %, at least one selected from the group consistingof Al: 0.01% or more and 2.00% or less, Nb: 0.005% or more and 0.200% orless, B: 0.0003% or more and 0.0050% or less, Ni: 0.005% or more and1.000% or less, Cr: 0.005% or more and 1.000% or less, V: 0.005% or moreand 0.500% or less, Mo: 0.005% or more and 1.000% or less, Cu: 0.005% ormore and 1.000% or less, Sn: 0.002% or more and 0.200% or less, Sb:0.002% or more and 0.200% or less, Ta: 0.001% or more and 0.010% orless, Ca: 0.0005% or more and 0.0050% or less, Mg: 0.0005% or more and0.0050% or less, and REM: 0.0005% or more and 0.0050% or less, with thebalance consisting of Fe and inevitable impurities; (ii) hot rolling thesteel slab with a finisher delivery temperature of 750° C. or higher and1000° C. or lower to obtain a steel sheet; (iii) coiling the steelsheet; (iv) then subjecting the steel sheet to pickling to removescales; (v) retaining the steel sheet in a temperature range of Ac₁[transformation temperature+20° C.] to [Ac₁ transformationtemperature+120° C.] for 600 s to 21,600 s; (vi) optionally cold rollingthe steel sheet at a rolling reduction of less than 30%; and (vii) thenretaining the steel sheet in a temperature range of [Ac₁ transformationtemperature+10° C.] to [Ac₁ transformation temperature+100° C.] for 20 sto 900 s and subsequently cooling the steel sheet.
 19. A method formanufacturing the steel sheet according to claim 11, the methodcomprising: (i) heating a steel slab having a chemical compositioncontaining, in mass %, C: 0.030% or more and 0.250% or less, Si: 0.01%or more and 3.00% or less, Mn: more than 4.20% and 6.00% or less, P:0.001% or more and 0.100% or less, S: 0.0200% or less, N: 0.0100% orless, and Ti: 0.005% or more and 0.200% or less, and optionally furthercontaining, in mass %, at least one selected from the group consistingof Al: 0.01% or more and 2.00% or less, Nb: 0.005% or more and 0.200% orless, B: 0.0003% or more and 0.0050% or less, Ni: 0.005% or more and1.000% or less, Cr: 0.005% or more and 1.000% or less, V: 0.005% or moreand 0.500% or less, Mo: 0.005% or more and 1.000% or less, Cu: 0.005% ormore and 1.000% or less, Sn: 0.002% or more and 0.200% or less, Sb:0.002% or more and 0.200% or less, Ta: 0.001% or more and 0.010% orless, Ca: 0.0005% or more and 0.0050% or less, Mg: 0.0005% or more and0.0050% or less, and REM: 0.0005% or more and 0.0050% or less, with thebalance consisting of Fe and inevitable impurities; (ii) hot rolling thesteel slab with a finisher delivery temperature of 750° C. or higher and1000° C. or lower to obtain a steel sheet; (iii) coiling the steelsheet; (iv) then subjecting the steel sheet to pickling to removescales; (v) retaining the steel sheet in a temperature range of Ac₁[transformation temperature+20° C.] to [Ac₁ transformationtemperature+120° C.] for 600 s to 21,600 s; (vi) optionally cold rollingthe steel sheet at a rolling reduction of less than 30%; and (vii) thenretaining the steel sheet in a temperature range of [Ac₁ transformationtemperature+10° C.] to [Ac₁ transformation temperature+100° C.] for 20 sto 900 s and subsequently cooling the steel sheet.
 20. The methodaccording to claim 16, wherein a value obtained by dividing a volumefraction of the retained austenite after performing tensile working withan elongation value of 10% by a volume fraction of the retainedaustenite before the tensile working equals 0.3 or more.
 21. The methodaccording to claim 17, wherein a value obtained by dividing a volumefraction of the retained austenite after performing tensile working withan elongation value of 10% by a volume fraction of the retainedaustenite before the tensile working equals 0.3 or more.
 22. The methodaccording to claim 18, wherein a value obtained by dividing a volumefraction of the retained austenite after performing tensile working withan elongation value of 10% by a volume fraction of the retainedaustenite before the tensile working equals 0.3 or more.
 23. The methodaccording to claim 19, wherein a value obtained by dividing a volumefraction of the retained austenite after performing tensile working withan elongation value of 10% by a volume fraction of the retainedaustenite before the tensile working equals 0.3 or more.
 24. A methodfor manufacturing the coated steel sheet according to claim 12,comprising: (i) heating a steel slab having a chemical compositioncontaining, in mass %, C: 0.030% or more and 0.250% or less, Si: 0.01%or more and 3.00% or less, Mn: more than 4.20% and 6.00% or less, P:0.001% or more and 0.100% or less, S: 0.0200% or less, N: 0.0100% orless, and Ti: 0.005% or more and 0.200% or less, and optionally furthercontaining, in mass %, at least one selected from the group consistingof Al: 0.01% or more and 2.00% or less, Nb: 0.005% or more and 0.200% orless, B: 0.0003% or more and 0.0050% or less, Ni: 0.005% or more and1.000% or less, Cr: 0.005% or more and 1.000% or less, V: 0.005% or moreand 0.500% or less, Mo: 0.005% or more and 1.000% or less, Cu: 0.005% ormore and 1.000% or less, Sn: 0.002% or more and 0.200% or less, Sb:0.002% or more and 0.200% or less, Ta: 0.001% or more and 0.010% orless, Ca: 0.0005% or more and 0.0050% or less, Mg: 0.0005% or more and0.0050% or less, and REM: 0.0005% or more and 0.0050% or less, with thebalance consisting of Fe and inevitable impurities; (ii) hot rolling thesteel slab with a finisher delivery temperature of 750° C. or higher and1000° C. or lower to obtain a steel sheet; (iii) coiling the steelsheet; (iv) then subjecting the steel sheet to pickling to removescales; (v) retaining the steel sheet in a temperature range of Ac₁[transformation temperature+20° C.] to [Ac₁ transformationtemperature+120° C.] for 600 s to 21,600 s; (vi) optionally cold rollingthe steel sheet at a rolling reduction of less than 30%; and (vii) thenretaining the steel sheet in a temperature range of [Ac₁ transformationtemperature+10° C.] to [Ac₁ transformation temperature+100° C.] for 20 sto 900 s and subsequently cooling the steel sheet: and (viii)thereafter, either subjecting the steel sheet after cooling to oneselected from hot-dip galvanizing treatment, hot-dip aluminum coatingtreatment, and electrogalvanizing treatment, or subjecting the steelsheet after cooling to hot-dip galvanizing treatment and then toalloying treatment at 450° C. or higher and 600° C. or lower.
 25. Amethod for manufacturing the coated steel sheet according to claim 13,comprising; (i) heating a steel slab having a chemical compositioncontaining, in mass %, C: 0.030% or more and 0.250% or less, Si: 0.01%or more and 3.00% or less, Mn: more than 4.20% and 6.00% or less, P:0.001% or more and 0.100% or less, S: 0.0200% or less, N: 0.0100% orless, and Ti: 0.005% or more and 0.200% or less, and optionally furthercontaining, in mass %, at least one selected from the group consistingof Al: 0.01% or more and 2.00% or less, Nb: 0.005% or more and 0.200% orless, B: 0.0003% or more and 0.0050% or less, Ni: 0.005% or more and1.000% or less, Cr: 0.005% or more and 1.000% or less, V: 0.005% or moreand 0.500% or less, Mo: 0.005% or more and 1.000% or less, Cu: 0.005% ormore and 1.000% or less, Sn: 0.002% or more and 0.200% or less, Sb:0.002% or more and 0.200% or less, Ta: 0.001% or more and 0.010% orless, Ca: 0.0005% or more and 0.0050% or less, Mg: 0.0005% or more and0.0050% or less, and REM: 0.0005% or more and 0.0050% or less, with thebalance consisting of Fe and inevitable impurities; (ii) hot rolling thesteel slab with a finisher delivery temperature of 750° C. or higher and1000° C. or lower to obtain a steel sheet; (iii) coiling the steelsheet; (iv) then subjecting the steel sheet to pickling to removescales; (v) retaining the steel sheet in a temperature range of Ac₁[transformation temperature+20° C.] to [Ac₁ transformationtemperature+120° C.] for 600 s to 21,600 s; (vi) optionally cold rollingthe steel sheet at a rolling reduction of less than 30%; and (vii) thenretaining the steel sheet in a temperature range of [Ac₁ transformationtemperature+10° C.] to [Ac₁ transformation temperature+100° C.] for 20 sto 900 s and subsequently cooling the steel sheet: and (viii)thereafter, either subjecting the steel sheet after cooling to oneselected from hot-dip galvanizing treatment, hot-dip aluminum coatingtreatment, and electrogalvanizing treatment, or subjecting the steelsheet after cooling to hot-dip galvanizing treatment and then toalloying treatment at 450° C. or higher and 600° C. or lower.
 26. Amethod for manufacturing the coated steel sheet according to claim 14,comprising: (i) heating a steel slab having a chemical compositioncontaining, in mass %, C: 0.030% or more and 0.250% or less, Si: 0.01%or more and 3.00% or less, Mn: more than 4.20% and 6.00% or less, P:0.001% or more and 0.100% or less, S: 0.0200% or less, N: 0.0100% orless, and Ti: 0.005% or more and 0.200% or less, and optionally furthercontaining, in mass %, at least one selected from the group consistingof Al: 0.01% or more and 2.00% or less, Nb: 0.005% or more and 0.200% orless, B: 0.0003% or more and 0.0050% or less, Ni: 0.005% or more and1.000% or less, Cr: 0.005% or more and 1.000% or less, V: 0.005% or moreand 0.500% or less, Mo: 0.005% or more and 1.000% or less, Cu: 0.005% ormore and 1.000% or less, Sn: 0.002% or more and 0.200% or less, Sb:0.002% or more and 0.200% or less, Ta: 0.001% or more and 0.010% orless, Ca: 0.0005% or more and 0.0050% or less, Mg: 0.0005% or more and0.0050% or less, and REM: 0.0005% or more and 0.0050% or less, with thebalance consisting of Fe and inevitable impurities; (ii) hot rolling thesteel slab with a finisher delivery temperature of 750° C. or higher and1000° C. or lower to obtain a steel sheet; (iii) coiling the steelsheet; (iv) then subjecting the steel sheet to pickling to removescales; (v) retaining the steel sheet in a temperature range of Ac₁[transformation temperature+20° C.] to [Ac₁ transformationtemperature+120° C.] for 600 s to 21,600 s; (vi) optionally cold rollingthe steel sheet at a rolling reduction of less than 30%; and (vii) thenretaining the steel sheet in a temperature range of [Ac₁ transformationtemperature+10° C.] to [Ac₁ transformation temperature+100° C.] for 20 sto 900 s and subsequently cooling the steel sheet: and (viii)thereafter, either subjecting the steel sheet after cooling to oneselected from hot-dip galvanizing treatment, hot-dip aluminum coatingtreatment, and electrogalvanizing treatment, or subjecting the steelsheet after cooling to hot-dip galvanizing treatment and then toalloying treatment at 450° C. or higher and 600° C. or lower.
 27. Amethod for manufacturing the coated steel sheet according to claim 15,comprising: (i) heating a steel slab having a chemical compositioncontaining, in mass %, C: 0.030% or more and 0.250% or less, Si: 0.01%or more and 3.00% or less, Mn: more than 4.20% and 6.00% or less, P:0.001% or more and 0.100% or less, S: 0.0200% or less, N: 0.0100% orless, and Ti: 0.005% or more and 0.200% or less, and optionally furthercontaining, in mass %, at least one selected from the group consistingof Al: 0.01% or more and 2.00% or less, Nb: 0.005% or more and 0.200% orless, B: 0.0003% or more and 0.0050% or less, Ni: 0.005% or more and1.000% or less, Cr: 0.005% or more and 1.000% or less, V: 0.005% or moreand 0.500% or less, Mo: 0.005% or more and 1.000% or less, Cu: 0.005% ormore and 1.000% or less, Sn: 0.002% or more and 0.200% or less, Sb:0.002% or more and 0.200% or less, Ta: 0.001% or more and 0.010% orless, Ca: 0.0005% or more and 0.0050% or less, Mg: 0.0005% or more and0.0050% or less, and REM: 0.0005% or more and 0.0050% or less, with thebalance consisting of Fe and inevitable impurities; (ii) hot rolling thesteel slab with a finisher delivery temperature of 750° C. or higher and1000° C. or lower to obtain a steel sheet; (iii) coiling the steelsheet; (iv) then subjecting the steel sheet to pickling to removescales; (v) retaining the steel sheet in a temperature range of Ac₁[transformation temperature+20° C.] to [Ac₁ transformationtemperature+120° C.] for 600 s to 21,600 s; (vi) optionally cold rollingthe steel sheet at a rolling reduction of less than 30%; and (vii) thenretaining the steel sheet in a temperature range of [Ac₁ transformationtemperature+10° C.] to [Ac₁ transformation temperature+100° C.] for 20 sto 900 s and subsequently cooling the steel sheet: and (viii)thereafter, either subjecting the steel sheet after cooling to oneselected from hot-dip galvanizing treatment, hot-dip aluminum coatingtreatment, and electrogalvanizing treatment, or subjecting the steelsheet after cooling to hot-dip galvanizing treatment and then toalloying treatment at 450° C. or higher and 600° C. or lower.